United States Army
The United States Army is the land warfare service branch of the United States Armed Forces. It is one of the seven uniformed services of the United States, is designated as the Army of the United States in the United States Constitution; as the oldest and most senior branch of the U. S. military in order of precedence, the modern U. S. Army has its roots in the Continental Army, formed to fight the American Revolutionary War —before the United States of America was established as a country. After the Revolutionary War, the Congress of the Confederation created the United States Army on 3 June 1784 to replace the disbanded Continental Army; the United States Army considers itself descended from the Continental Army, dates its institutional inception from the origin of that armed force in 1775. As a uniformed military service, the U. S. Army is part of the Department of the Army, one of the three military departments of the Department of Defense; the U. S. Army is headed by a civilian senior appointed civil servant, the Secretary of the Army and by a chief military officer, the Chief of Staff of the Army, a member of the Joint Chiefs of Staff.
It is the largest military branch, in the fiscal year 2017, the projected end strength for the Regular Army was 476,000 soldiers. S. Army was 1,018,000 soldiers; as a branch of the armed forces, the mission of the U. S. Army is "to fight and win our Nation's wars, by providing prompt, land dominance, across the full range of military operations and the spectrum of conflict, in support of combatant commanders"; the branch participates in conflicts worldwide and is the major ground-based offensive and defensive force of the United States. The United States Army serves as the land-based branch of the U. S. Armed Forces. Section 3062 of Title 10, U. S. Code defines the purpose of the army as: Preserving the peace and security and providing for the defense of the United States, the Commonwealths and possessions and any areas occupied by the United States Supporting the national policies Implementing the national objectives Overcoming any nations responsible for aggressive acts that imperil the peace and security of the United StatesIn 2018, the Army Strategy 2018 articulated an eight-point addendum to the Army Vision for 2028.
While the Army Mission remains constant, the Army Strategy builds upon the Army's Brigade Modernization by adding focus to Corps and Division-level echelons. Modernization, reform for high-intensity conflict, Joint multi-domain operations are added to the strategy, to be completed by 2028; the Continental Army was created on 14 June 1775 by the Second Continental Congress as a unified army for the colonies to fight Great Britain, with George Washington appointed as its commander. The army was led by men who had served in the British Army or colonial militias and who brought much of British military heritage with them; as the Revolutionary War progressed, French aid and military thinking helped shape the new army. A number of European soldiers came on their own to help, such as Friedrich Wilhelm von Steuben, who taught Prussian Army tactics and organizational skills; the army fought numerous pitched battles and in the South in 1780–1781, at times using the Fabian strategy and hit-and-run tactics, under the leadership of Major General Nathanael Greene, hit where the British were weakest to wear down their forces.
Washington led victories against the British at Trenton and Princeton, but lost a series of battles in the New York and New Jersey campaign in 1776 and the Philadelphia campaign in 1777. With a decisive victory at Yorktown and the help of the French, the Continental Army prevailed against the British. After the war, the Continental Army was given land certificates and disbanded in a reflection of the republican distrust of standing armies. State militias became the new nation's sole ground army, with the exception of a regiment to guard the Western Frontier and one battery of artillery guarding West Point's arsenal. However, because of continuing conflict with Native Americans, it was soon realized that it was necessary to field a trained standing army; the Regular Army was at first small and after General St. Clair's defeat at the Battle of the Wabash, the Regular Army was reorganized as the Legion of the United States, established in 1791 and renamed the United States Army in 1796; the War of 1812, the second and last war between the United States and Great Britain, had mixed results.
The U. S. Army did not conquer Canada but it did destroy Native American resistance to expansion in the Old Northwest and it validated its independence by stopping two major British invasions in 1814 and 1815. After taking control of Lake Erie in 1813, the U. S. Army seized parts of western Upper Canada, burned York and defeated Tecumseh, which caused his Western Confederacy to collapse. Following U. S. victories in the Canadian province of Upper Canada, British troops who had dubbed the U. S. Army "Regulars, by God!", were able to capture and burn Washington, defended by militia, in 1814. The regular army, however proved they were professional and capable of defeating the British army during the invasions of Plattsburgh and Baltimore, prompting British agreement on the rejected terms of a status quo ante bellum. Two weeks after a treaty was signed, Andrew Jackson defeated the British in the Battle of New Orleans and Siege of Fort St. Philip, became a national hero. U. S. troops and sailors captured HMS Cyane and Penguin in the final engagements of the war.
Per the treaty, both sides (the United S
The Alpine Wall was an Italian system of fortifications along the 1851 km of Italy's northern frontier. Built in the years leading up to World War II at the direction of Italian dictator Benito Mussolini, the defensive line faced France, Switzerland and Yugoslavia, it was defended by Italian special troops. The Alpine line was similar in concept to other fortifications of the same era, including the Maginot Line of France, the Siegfried Line of Germany, the National Redoubt of Switzerland. Italy's land frontiers were in most places mountainous and defended, but in the years leading up to World War II, Italy's relations with its neighbours were uneasy. In its dealings with its German ally, Italy was concerned about German ambitions towards the province of South Tyrol, inhabited by a German majority. Due to the rugged nature of the Alpine frontier, defences were confined to passes and observation posts in accessible locations. Work on the Alpine Wall began in 1931, intended to cover an arc from the Mediterranean coast at Ventimiglia in the west to Fiume on the Adriatic coast in the east.
Three zones were designated at increasing distances from the frontier: "Zone of Security": Initial contact with the enemy. "Zone of Resistance": Heavier fortifications capable of resistance in isolation. "Zone of Alignment": Assembly area for counterattack, into which the enemy was to be directed. Three types of fortifications were provided: "Type A": The largest fortifications built into mountainsides. "Type B": Smaller point-defence fortifications. "Type C": Widely distributed shelters and rallying pointsThe work, carried out in secrecy using Italian labor, was a significant economic burden, resulting in 208 opere with 647 machine guns and fifty artillery pieces. Construction continued until 1942; the forts were armed with a mixture of new weaponry and older equipment from World War I. Provisions were made to deal with the use of poison gas. Much of the armour was obtained from Germany in compensation for Italian military ventures on behalf of the Axis. Little use was made of the Alpine Wall during World War II.
During the Italian invasion of France in 1940, some western forts such as Fort Chaberton exchanged fire with their French counterparts of the Alpine Line. Chaberton suffered disabling damage. In addition, some Alpine Wall fortifications were used defensively by the Germans in 1944. At the end of the conflict, some of the western fortifications were destroyed. In the east former Austro-Hungarian territories that Italy acquired with the London treaty of 1915 were in 1947 awarded to Yugoslavia; the entire eastern part of the pre-WW2 fortifications were thus on the Yugoslav and are now on Slovenian territory. The 1947 Paris Peace Treaty forbade the construction or expansion of fortifications within twenty kilometers of the border. However, with Italy's membership in NATO, construction began on a new defensive line from Austria to the Adriatic along the Yugoslavian border along the Natisone and Tagliamento rivers; the new line used tank turrets in a manner similar to German defences during the previous conflict, allowing 360 degree traverse and a high rate of fire.
By 1976 this system was still considered useful in any conflict short of nuclear war. The end of the Cold War brought an end to the usefulness of the Alpine Wall; the emplacements were stripped and sealed in 1991-1992. Only some active fortifications have been preserved; the fortifications were constructed in the flanking heights of the valleys, with works within the valleys only where they were sufficiently wide. Anti-tank guns and machine guns were trained on prepared fields of fire, with observation stations at higher points. Shelters for infantry were located rearwards. A system of communications links and roads, or for higher locations, ropeways were provided for communication and supply; the individual fortifications were built in rock on valley sides. Where this was not suitable, concrete was used for protection, with a minimum of openings and three to five metres of concrete thickness. Combat blocks were with ammunition rooms behind. Underground galleries connected the combat blocks and their support areas, such as the utility rooms, barracks and command centres, with the main entry farthest to the rear.
Combat areas were isolated from the rest of the structure by gas-tight doorways. Units built after 1939 were designed to operate independently, cut off from supplies. Fortifications were camouflaged so that they appeared to blend with the surroundings, whether doors or embrasures were open or closed. Emergency escape routes were provided. Armament included an anti-tank gun and a number of machine guns. Post-war units used tank turrets. Usual armament included: Machine gun, Fiat 35 in casemate or metal turret Machine gun, Breda 30 and Breda M37, for defense of entries to the fortifications Gun 57/43 RM mod. 887 on naval mounts Gun 75/43, ball mount in 10 cm steel slab Gun 47/32 Mortar 81mm mod. 35 FlamethrowerFortifications were surrounded by minefields and barbed wire. Where feasible, an anti-tank ditch was provided; the Vallo Alpino was defended by the 21,000 strong "Guardia alla Frontiera", a special Italian Corp created in 1937. They defended the 1,851 km of northern Italian frontiers with the so-called "Vallo Alpino Occidentale", "Vallo Alpino Settentrionale" and "Vallo Alpino Orientale".
Atlantic Wall Czechoslovak border fortifications Marco Boglione, Le Strade dei Cannoni Blu Edizioni, Torino 2005, ISBN 88-87417-68-7 Kauffmann, J
Socialist Federal Republic of Yugoslavia
The Socialist Federal Republic of Yugoslavia was a country located in central and Southeastern Europe that existed from its foundation in the aftermath of World War II until its dissolution in 1992 amid the Yugoslav Wars. Covering an area of 255,804 km², the SFRY was bordered by the Adriatic Sea and Italy to the west and Hungary to the north and Romania to the east, Albania and Greece to the south; the nation was a socialist state and a federation governed by the League of Communists of Yugoslavia and made up of six socialist republics: Bosnia and Herzegovina, Macedonia, Montenegro and Slovenia with Belgrade as its capital. In addition, it included two autonomous provinces within Serbia: Vojvodina; the SFRY's origin is traced to 26 November 1942, when the Anti-Fascist Council for the National Liberation of Yugoslavia was formed during World War II. On 29 November 1945, the Federal People's Republic of Yugoslavia was proclaimed after the deposition of King Peter II, thus ending the monarchy.
Until 1948, the new communist government sided with the Eastern Bloc under the leadership of Josip Broz Tito at the beginning of the Cold War, but after the Tito–Stalin split of 1948, Yugoslavia pursued a policy of neutrality. It became one of the founding members of the Non-Aligned Movement, transitioned from a planned economy to market socialism; the SFRY maintained neutrality during the Cold War as part of its foreign policy. It was a founding member of CERN, the United Nations, Non-Aligned Movement, OSCE, IFAD, WTO, BTWC. Following the death of Tito on 4 May 1980, the Yugoslav economy started to collapse, which increased unemployment and inflation; the economic crisis led to a rise in ethnic nationalism in early 1990s. With the collapse of communism in Eastern Europe, inter-republic talks on transformation of the federation failed. In 1991 some European states recognized their independence; the federation collapsed along federal borders, followed by the start of the Yugoslav Wars, the final downfall and breakup of the federation on 27 April 1992.
Two of its republics and Montenegro, remained within a reconstituted state known as the "Federal Republic of Yugoslavia", but this union was not recognized internationally as the official successor state to the SFRY. The term "former Yugoslavia" is now used retrospectively; the name Yugoslavia, an Anglicised transcription of Jugoslavija, is a composite word made up of jug and slavija. The Slavic word jug means'south', while slavija denotes a'land of the Slavs'. Thus, a translation of Jugoslavija would be'South-Slavia' or'Land of the South Slavs'; the full official name of the federation varied between 1945 and 1992. Yugoslavia was formed in 1918 under the name Kingdom of Serbs and Slovenes. In January 1929, King Alexander I assumed dictatorship of the kingdom and renamed it the Kingdom of Yugoslavia, for the first time making the term "Yugoslavia"—which had been used colloquially for decades —the official name of the state. After the Kingdom was occupied by the Axis during World War II, the Anti-Fascist Council for the National Liberation of Yugoslavia announced in 1943 the formation of the Democratic Federal Yugoslavia in the substantial resistance-controlled areas of the country.
The name deliberately left the republic-or-kingdom question open. In 1945, King Peter II was deposed, with the state reorganized as a republic, accordingly renamed Federal People's Republic of Yugoslavia, with the constitution coming into force in 1946. In 1963, amid pervasive liberal constitutional reforms, the name Socialist Federal Republic of Yugoslavia was introduced; the state is most referred to by the latter name, which it held for the longest period of all. Of the three main Yugoslav languages, the Serbo-Croatian and Macedonian language name for the state was identical, while Slovene differed in capitalization and the spelling of the adjective "Socialist"; the names are as follows: Serbo-Croatian and Macedonian languages Latin: Socijalistička Federativna Republika Jugoslavija Cyrillic: Социјалистичка Федеративна Република Југославија Serbo-Croatian pronunciation: Macedonian pronunciation: Slovene language Socialistična federativna republika Jugoslavija Due to the length of the name, abbreviations were used to refer to the Socialist Federal Republic of Yugoslavia, though the state was most known as Yugoslavia.
The most common abbreviation is SFRY, though SFR Yugoslavia was used in an official capacity by the media. On 6 April 1941, Yugoslavia was invaded by the Axis powers led by Nazi Germany. Yugoslav resistance was soon established in two forms, the Royal Yugoslav Army in the Homeland and the Communist Yugoslav Partisans; the Partisan supreme commander was Josip Broz Tito, under his command the movement soon began establishing "liberated territories" which attracted the attention of occupying forces. Unlike the various nationalist militias operating in occupied Yugoslavia, the Partisans were a pan-Yugoslav movement promoting the "brotherhood and unity" of Yugoslav nations, representing the republican, left-wing, socialist elements of the Yugoslav political
A land mine is an explosive device concealed under or on the ground and designed to destroy or disable enemy targets, ranging from combatants to vehicles and tanks, as they pass over or near it. Such a device is detonated automatically by way of pressure when a target steps on it or drives over it, although other detonation mechanisms are sometimes used. A land mine may cause damage by direct blast effect, by fragments that are thrown by the blast, or by both; the name originates from the ancient practice of military mining, where tunnels were dug under enemy fortifications or troop formations. These killing tunnels were at first collapsed to destroy targets located above, but they were filled with explosives and detonated in order to cause greater devastation. Nowadays, in common parlance, "land mine" refers to devices manufactured as anti-personnel or anti-vehicle weapons. Though some types of improvised explosive devices are mistakenly classified as land mines, the term land mine is reserved for manufactured devices designed to be used by recognized military services, whereas IED is used for makeshift "devices placed or fabricated in an improvised manner incorporating explosive material, lethal, incendiary, pyrotechnic materials or chemicals designed to destroy, distract or harass.
They may incorporate military stores, but are devised from non-military components". The use of land mines is controversial because of their potential as indiscriminate weapons, they can remain dangerous many years after a conflict has ended, harming the economy. 78 countries are contaminated with land mines and 15,000–20,000 people are killed every year while countless more are maimed. 80% of land mine casualties are civilian, with children as the most affected age group. Most killings occur in times of peace. With pressure from a number of campaign groups organised through the International Campaign to Ban Landmines, a global movement to prohibit their use led to the 1997 Convention on the Prohibition of the Use, Stockpiling and Transfer of Anti-Personnel Mines and on their Destruction known as the Ottawa Treaty. To date, 164 nations have signed the treaty. Land mines were designed for two main uses: To create defensive tactical barriers, channelling attacking forces into predetermined fire zones or slowing an invading force's progress to allow reinforcements to arrive.
To act as passive area-denial weapons. Land mines are used in large quantities for this first purpose, thus their widespread use in the demilitarized zones of flashpoints such as Cyprus and Korea; as of 2013, the only governments that still laid land mines were Myanmar in its internal conflict, Syria in its civil war. Land mines continue to kill or injure at least 4,300 people every year decades after the ends of the conflicts for which they were placed. Jiao Yu in the preface to his Huolongjing Quanzhi, written in 1412 AD, claimed that in the third century, the chancellor Zhuge Liang of the Shu Han state had used not only "fire weapons" but land mines in the Battle of Hulugu Valley against the forces of Sima Yi and his son Sima Zhao of the rival Cao Wei state; this claim is dubious, as gunpowder warfare did not develop in China until the advent of the flamethrower in the 10th century, while the land mine was not seen in China until the late 13th century. Explosive land mines were used in 1277 by the Chinese during the Song dynasty against an assault of the Mongols, who were besieging a city in southern China.
The invention of this detonated "enormous bomb" was credited to one Lou Qianxia of the 13th century. The famous 14th-century Chinese text of the Huolongjing, the first to describe hollow cast iron cannonball shells filled with gunpowder, was the first to describe the invention of the land mine in greater detail than references found in texts written beforehand; this mid 14th century work compiled during the late Yuan dynasty and early Ming dynasty stated that mines were made of cast iron and were spherical in shape, filled with either "magic gunpowder", "poison gunpowder", or "blinding and burning gunpowder", any one of these compositions being suitable for use. The wad of the mine was made of hard wood, carrying three different fuses in case of defective connection to the touch hole. In those days, the Chinese relied upon command signals and timed calculation of enemy movements into the minefield, since a long fuse had to be ignited by hand from the ambushers in a somewhat far-off location lying in wait.
The Huolongjing describes land mines that were set off by enemy movement, called the'ground-thunder explosive camp', one of the'self-trespassing' types, as the text says: These mines are installed at frontier gates and passes. Pieces of bamboo are sawn into sections nine feet in length, all septa in the bamboo being removed, save only the last. Boiling oil is next left there for some time before being removed; the fuse starts from the bottom, is compressed into it to form an explosive mine. The gunpowder fills up eight-tenths of the tube, while lead or iron pellets take up the rest of the space. A trench five feet in depth is dug; the fuse is connected to a firing device. The Huolongjing describes the trigger device used for this as a "steel wheel", which directed sparks
A caltrop is an antipersonnel weapon made up of two or more sharp nails or spines arranged in such a manner that one of them always points upward from a stable base. Caltrops were part of defences that served to slow the advance of troops on foot, war elephants, were effective against the soft feet of camels. In modern times, caltrops are effective when used against wheeled vehicles with pneumatic tires; the modern name "caltrop" is derived from the Latin calcitrapa, such as in the French usage chausse-trape. The synonymous Latin word tribulus provides part of the modern Latin name of a plant called the caltrop, Tribulus terrestris, whose spiked seed case can injure feet and puncture tires; this plant can be compared to Centaurea calcitrapa, sometimes referred to as the "caltrop". A water plant with similarly-shaped spiked seeds is called the "water caltrop", Trapa natans. Caltrops were known to the Romans as tribulus or sometimes as murex ferreus, the latter meaning'jagged iron'. In Ancient Greek the word tribalos meant three spikes.
They were used in the Battle of Carrhae in 53 BC. The late Roman writer Vegetius, referring in his work De Re Militari to scythed chariots, wrote: The armed chariots used in war by Antiochus and Mithridates at first terrified the Romans, but they afterwards made a jest of them; as a chariot of this sort does not always meet with plain and level ground, the least obstruction stops it. And if one of the horses be either killed or wounded, it falls into the enemy's hands; the Roman soldiers rendered them useless chiefly by the following contrivance: at the instant the engagement began, they strewed the field of battle with caltrops, the horses that drew the chariots, running full speed on them, were infallibly destroyed. A caltrop is a device composed of four spikes or points arranged so that in whatever manner it is thrown on the ground, it rests on three and presents the fourth upright. Another example of the use of caltrops was found in Jamestown, Virginia, in the United States: Undoubtedly the most unusual weapon or military device surviving from seventeenth-century Virginia is known as a caltrop, a single example of, found at Jamestown.
It amounts to a spread iron tripod about three inches long with another leg sticking vertically upward, so that however you throw it down, one spike always sticks up.... There is no doubt that the most inscrutable Indian treading on a caltrop would be shocked into noisy comment.... The fact that only one has been found would seem to suggest that they were used little, if at all; as with all military equipment designed for European wars, the caltrop's presence in Virginia must be considered in the light of possible attacks by the Spaniards as well as assaults from the Indians. The Japanese version of the caltrop is called makibishi. Makibishi were sharp spiked objects that were used in feudal Japan to slow pursuers and were used in the defence of samurai fortifications. Iron makibishi were called tetsubishi, while the makibishi made from the dried seed pod of the'water caltrop' or'water chestnut', formed a natural type of makibashi called tennenbishi. Both types of makibishi could penetrate the thin soles of shoes, such as the waraji sandals, that were worn in feudal Japan.
Punji sticks perform a similar role to caltrops. These are sharpened sticks placed vertically in the ground, their use in modern times targets the body and limbs of a falling victim by means of a pit or tripwire. During the Second World War, large caltrop-shaped objects made from reinforced concrete were used as anti-tank devices, although it seems that these were rare. Much more common were concrete devices called dragon's teeth, which were designed to wedge into tank treads. Large ones weighing over 1 tonne are still used defensively by the army to deny access to wheeled vehicles in camp areas; as dragon's teeth are immobile, the analogy with the caltrop is inexact. Another caltrop-like defence during World War II was the massive steel, freestanding Czech hedgehog; these were used by the Germans to defend beaches at Normandy and other coastal areas. Tetrapods are concrete blocks shaped like caltrops, which interlock when piled up, they are used in the construction of breakwaters and other sea defences, as they have been found to let the water pass through them and interrupt natural processes less than some other defenses.
Researchers have tried to develop a caltrop-like device to deflate vehicle tires in a manner useful to law enforcement agencies or the military. During service in World War I, Australian Light Horse troops collected caltrops as keepsakes; these caltrops were either made by welding two pieces of wire together to form a four-pointed star or pouring molten steel into a mould to form a solid, seven-pointed star. The purpose of these devices was to disable horses, they were exchanged with French troops for bullets. The Australian Light Horse troops referred to them as "Horse Chestnuts". Caltrops were used extensively and during World War II; the modifications and variants produced by the Special Operations Executive and the Office of Strategic Services of the United States are still in use today within special forces and law enforcement bodies. The Germans dropped crowsfeet; these were made from two segments of sheet metal welded together into a tetrapod with four barbed points and painted in camouflage colours.
They came in two sizes with a side length of either 3
A fortification is a military construction or building designed for the defense of territories in warfare, is used to solidify rule in a region during peacetime. The term is derived from the Latin fortis and facere. From early history to modern times, walls have been necessary for cities to survive in an ever-changing world of invasion and conquest; some settlements in the Indus Valley Civilization were the first small cities to be fortified. In ancient Greece, large stone walls had been built in Mycenaean Greece, such as the ancient site of Mycenae. A Greek phrourion was a fortified collection of buildings used as a military garrison, is the equivalent of the Roman castellum or English fortress; these constructions served the purpose of a watch tower, to guard certain roads and lands that might threaten the kingdom. Though smaller than a real fortress, they acted as a border guard rather than a real strongpoint to watch and maintain the border; the art of setting out a military camp or constructing a fortification traditionally has been called "castrametation" since the time of the Roman legions.
Fortification is divided into two branches: permanent fortification and field fortification. There is an intermediate branch known as semi-permanent fortification. Castles are fortifications which are regarded as being distinct from the generic fort or fortress in that they are a residence of a monarch or noble and command a specific defensive territory. Roman forts and hill forts were the main antecedents of castles in Europe, which emerged in the 9th century in the Carolingian Empire; the Early Middle Ages saw the creation of some towns built around castles. Medieval-style fortifications were made obsolete by the arrival of cannons in the 14th century. Fortifications in the age of black powder evolved into much lower structures with greater use of ditches and earth ramparts that would absorb and disperse the energy of cannon fire. Walls exposed to direct cannon fire were vulnerable, so the walls were sunk into ditches fronted by earth slopes to improve protection; the arrival of explosive shells in the 19th century led to yet another stage in the evolution of fortification.
Star forts did not fare well against the effects of high explosive, the intricate arrangements of bastions, flanking batteries and the constructed lines of fire for the defending cannon could be disrupted by explosive shells. Steel-and-concrete fortifications were common during the early 20th centuries; however the advances in modern warfare since World War I have made large-scale fortifications obsolete in most situations. Demilitarized zones along borders are arguably another type of fortification, although a passive kind, providing a buffer between hostile militaries. Many US military installations are known as forts. Indeed, during the pioneering era of North America, many outposts on the frontiers non-military outposts, were referred to generically as forts. Larger military installations may be called fortresses; the word fortification can refer to the practice of improving an area's defence with defensive works. City walls are fortifications but are not called fortresses; the art of setting out a military camp or constructing a fortification traditionally has been called castrametation since the time of the Roman legions.
The art/science of laying siege to a fortification and of destroying it is called siegecraft or siege warfare and is formally known as poliorcetics. In some texts this latter term applies to the art of building a fortification. Fortification is divided into two branches: permanent fortification and field fortification. Permanent fortifications are erected at leisure, with all the resources that a state can supply of constructive and mechanical skill, are built of enduring materials. Field fortifications—for example breastworks—and known as fieldworks or earthworks, are extemporized by troops in the field assisted by such local labour and tools as may be procurable and with materials that do not require much preparation, such as earth and light timber, or sandbags. An example of field fortification was the construction of Fort Necessity by George Washington in 1754. There is an intermediate branch known as semi-permanent fortification; this is employed when in the course of a campaign it becomes desirable to protect some locality with the best imitation of permanent defences that can be made in a short time, ample resources and skilled civilian labour being available.
An example of this is the construction of Roman forts in England and in other Roman territories where camps were set up with the intention of staying for some time, but not permanently. Castles are fortifications which are regarded as being distinct from the generic fort or fortress in that it describes a residence of a monarch or noble and commands a specific defensive territory. An example of this is the massive medieval castle of Carcassonne. From early history to modern times, walls have been a necessity for many cities. In Bulgaria, near the town of Provadia a walled fortified settlement today called Solnitsata starting from 4700 BC had a diameter of about 300 feet, was home to 350 people living in two-storey houses, was encircled by a fortified wall; the huge walls around the settlement, which were built tall and with stone blocks which are 6 feet high and 4.5 feet thick, make it one of the earliest walled settlements in Europe but it is younger than the walled town of Sesklo in Greece from 6800 BC.
Uruk in ancient Su
Steel is an alloy of iron and carbon, sometimes other elements. Because of its high tensile strength and low cost, it is a major component used in buildings, tools, automobiles, machines and weapons. Iron is the base metal of steel. Iron is able to take on two crystalline forms, body centered cubic and face centered cubic, depending on its temperature. In the body-centered cubic arrangement, there is an iron atom in the center and eight atoms at the vertices of each cubic unit cell, it is the interaction of the allotropes of iron with the alloying elements carbon, that gives steel and cast iron their range of unique properties. In pure iron, the crystal structure has little resistance to the iron atoms slipping past one another, so pure iron is quite ductile, or soft and formed. In steel, small amounts of carbon, other elements, inclusions within the iron act as hardening agents that prevent the movement of dislocations that are common in the crystal lattices of iron atoms; the carbon in typical steel alloys may contribute up to 2.14% of its weight.
Varying the amount of carbon and many other alloying elements, as well as controlling their chemical and physical makeup in the final steel, slows the movement of those dislocations that make pure iron ductile, thus controls and enhances its qualities. These qualities include such things as the hardness, quenching behavior, need for annealing, tempering behavior, yield strength, tensile strength of the resulting steel; the increase in steel's strength compared to pure iron is possible only by reducing iron's ductility. Steel was produced in bloomery furnaces for thousands of years, but its large-scale, industrial use began only after more efficient production methods were devised in the 17th century, with the production of blister steel and crucible steel. With the invention of the Bessemer process in the mid-19th century, a new era of mass-produced steel began; this was followed by the Siemens–Martin process and the Gilchrist–Thomas process that refined the quality of steel. With their introductions, mild steel replaced wrought iron.
Further refinements in the process, such as basic oxygen steelmaking replaced earlier methods by further lowering the cost of production and increasing the quality of the final product. Today, steel is one of the most common manmade materials in the world, with more than 1.6 billion tons produced annually. Modern steel is identified by various grades defined by assorted standards organizations; the noun steel originates from the Proto-Germanic adjective stahliją or stakhlijan, related to stahlaz or stahliją. The carbon content of steel is between 0.002% and 2.14% by weight for plain iron–carbon alloys. These values vary depending on alloying elements such as manganese, nickel, so on. Steel is an iron-carbon alloy that does not undergo eutectic reaction. In contrast, cast iron does undergo eutectic reaction. Too little carbon content leaves iron quite soft and weak. Carbon contents higher than those of steel make a brittle alloy called pig iron. While iron alloyed with carbon is called carbon steel, alloy steel is steel to which other alloying elements have been intentionally added to modify the characteristics of steel.
Common alloying elements include: manganese, chromium, boron, vanadium, tungsten and niobium. Additional elements, most considered undesirable, are important in steel: phosphorus, sulfur and traces of oxygen and copper. Plain carbon-iron alloys with a higher than 2.1% carbon content are known as cast iron. With modern steelmaking techniques such as powder metal forming, it is possible to make high-carbon steels, but such are not common. Cast iron is not malleable when hot, but it can be formed by casting as it has a lower melting point than steel and good castability properties. Certain compositions of cast iron, while retaining the economies of melting and casting, can be heat treated after casting to make malleable iron or ductile iron objects. Steel is distinguishable from wrought iron, which may contain a small amount of carbon but large amounts of slag. Iron is found in the Earth's crust in the form of an ore an iron oxide, such as magnetite or hematite. Iron is extracted from iron ore by removing the oxygen through its combination with a preferred chemical partner such as carbon, lost to the atmosphere as carbon dioxide.
This process, known as smelting, was first applied to metals with lower melting points, such as tin, which melts at about 250 °C, copper, which melts at about 1,100 °C, the combination, which has a melting point lower than 1,083 °C. In comparison, cast iron melts at about 1,375 °C. Small quantities of iron were smelted in ancient times, in the solid state, by heating the ore in a charcoal fire and welding the clumps together with a hammer and in the process squeezing out the impurities. With care, the carbon content could be controlled by moving it around in the fire. Unlike copper and tin, liquid or solid iron dissolves carbon quite readily. All of these temperatures could be reached with ancient methods used since the Bronze Age. Since the oxidation rate of iron increases beyond 800 °C, it is important that smelting take place in a low-oxygen environment. Smelting, using carbon to reduce iro