Milan is a city in northern Italy, capital of Lombardy, the second-most populous city in Italy after Rome, with the city proper having a population of 1,372,810 while its metropolitan city has a population of 3,245,308. Its continuously built-up urban area has a population estimated to be about 5,270,000 over 1,891 square kilometres; the wider Milan metropolitan area, known as Greater Milan, is a polycentric metropolitan region that extends over central Lombardy and eastern Piedmont and which counts an estimated total population of 7.5 million, making it by far the largest metropolitan area in Italy and the 54th largest in the world. Milan served as capital of the Western Roman Empire from 286 to 402 and the Duchy of Milan during the medieval period and early modern age. Milan is considered a leading alpha global city, with strengths in the field of the art, design, entertainment, finance, media, services and tourism, its business district hosts Italy's stock exchange and the headquarters of national and international banks and companies.
In terms of GDP, it has the third-largest economy among European cities after Paris and London, but the fastest in growth among the three, is the wealthiest among European non-capital cities. Milan is considered part of the Blue Banana and one of the "Four Motors for Europe"; the city has been recognized as one of the world's four fashion capitals thanks to several international events and fairs, including Milan Fashion Week and the Milan Furniture Fair, which are among the world's biggest in terms of revenue and growth. It hosted the Universal Exposition in 1906 and 2015; the city hosts numerous cultural institutions and universities, with 11% of the national total enrolled students. Milan is the destination of 8 million overseas visitors every year, attracted by its museums and art galleries that boast some of the most important collections in the world, including major works by Leonardo da Vinci; the city is served by a large number of luxury hotels and is the fifth-most starred in the world by Michelin Guide.
The city is home to two of Europe's most successful football teams, A. C. Milan and F. C. Internazionale, one of Italy's main basketball teams, Olimpia Milano; the etymology of the name Milan remains uncertain. One theory holds that the Latin name Mediolanum planus. However, some scholars believe that lanum comes from the Celtic root lan, meaning an enclosure or demarcated territory in which Celtic communities used to build shrines. Hence Mediolanum could signify the central sanctuary of a Celtic tribe. Indeed, about sixty Gallo-Roman sites in France bore the name "Mediolanum", for example: Saintes and Évreux. In addition, another theory links the name to the boar sow an ancient emblem of the city, fancifully accounted for in Andrea Alciato's Emblemata, beneath a woodcut of the first raising of the city walls, where a boar is seen lifted from the excavation, the etymology of Mediolanum given as "half-wool", explained in Latin and in French; the foundation of Milan is credited to two Celtic peoples, the Bituriges and the Aedui, having as their emblems a ram and a boar.
Alciato credits Ambrose for his account. The Celtic Insubres, the inhabitants of the region of northern Italy called Insubria, appear to have founded Milan around 600 BC. According to the legend reported by Livy, the Gaulish king Ambicatus sent his nephew Bellovesus into northern Italy at the head of a party drawn from various Gaulish tribes; the Romans, led by consul Gnaeus Cornelius Scipio Calvus, fought the Insubres and captured the city in 222 BC. They conquered the entirety of the region, calling the new province "Cisalpine Gaul" – "Gaul this side of the Alps" – and may have given the site its Latinized Celtic name of Mediolanum: in Gaulish *medio- meant "middle, center" and the name element -lanon is the Celtic equivalent of Latin -planum "plain", thus *Mediolanon meant " in the midst of the plain". In 286 the Roman Emperor Diocletian moved the capital of the Western Roman Empire from Rome to Mediolanum. Diocletian himself chose to reside at Nicomedia in the Eastern Empire, leaving his colleague Maximian at Milan.
Maximian built several gigantic monuments, the large circus, the thermae or "Baths of Hercules", a large complex of imperial palaces and other services and buildings of which fewer visible traces remain. Maximian increased the city area surrounded by a new, larger stone wall encompassing an area of 375 acres with many 24-sided towers; the monumental area had twin towers. From Mediolanum the Emperor Constantine issued the Edict of Milan in 313 AD, granting tolerance to all religions within the Empire, thus paving the way for Christianity to become the dominant religion of Roman Europe. Constantine had come to Mediolanum to celebrate the wedding of his sister
A transmission is a machine in a power transmission system, which provides controlled application of the power. The term transmission refers to the gearbox that uses gears and gear trains to provide speed and torque conversions from a rotating power source to another device. In British English, the term transmission refers to the whole drivetrain, including clutch, prop shaft and final drive shafts. In American English, the term refers more to the gearbox alone, detailed usage differs; the most common use is in motor vehicles, where the transmission adapts the output of the internal combustion engine to the drive wheels. Such engines need to operate at a high rotational speed, inappropriate for starting and slower travel; the transmission reduces the higher engine speed to the slower wheel speed, increasing torque in the process. Transmissions are used on pedal bicycles, fixed machines, where different rotational speeds and torques are adapted. A transmission has multiple gear ratios with the ability to switch between them as speed varies.
This switching may be done automatically. Directional control may be provided. Single-ratio transmissions exist, which change the speed and torque of motor output. In motor vehicles, the transmission is connected to the engine crankshaft via a flywheel or clutch or fluid coupling because internal combustion engines cannot run below a particular speed; the output of the transmission is transmitted via the driveshaft to one or more differentials, which drives the wheels. While a differential may provide gear reduction, its primary purpose is to permit the wheels at either end of an axle to rotate at different speeds as it changes the direction of rotation. Conventional gear/belt transmissions are not the only mechanism for speed/torque adaptation. Alternative mechanisms include power transformation. Hybrid configurations exist. Automatic transmissions use a valve body to shift gears using fluid pressures in response to speed and throttle input. Early transmissions included the right-angle drives and other gearing in windmills, horse-powered devices, steam engines, in support of pumping and hoisting.
Most modern gearboxes are used to increase torque while reducing the speed of a prime mover output shaft. This means that the output shaft of a gearbox rotates at a slower rate than the input shaft, this reduction in speed produces a mechanical advantage, increasing torque. A gearbox can be set up to do the opposite and provide an increase in shaft speed with a reduction of torque; some of the simplest gearboxes change the physical rotational direction of power transmission. Many typical automobile transmissions include the ability to select one of several gear ratios. In this case, most of the gear ratios are used to slow down the output speed of the engine and increase torque. However, the highest gears may be "overdrive" types. Gearboxes have found use in a wide variety of different—often stationary—applications, such as wind turbines. Transmissions are used in agricultural, construction and automotive equipment. In addition to ordinary transmission equipped with gears, such equipment makes extensive use of the hydrostatic drive and electrical adjustable-speed drives.
The simplest transmissions called gearboxes to reflect their simplicity, provide gear reduction, sometimes in conjunction with a right-angle change in direction of the shaft. These are used on PTO-powered agricultural equipment, since the axial PTO shaft is at odds with the usual need for the driven shaft, either vertical, or horizontally extending from one side of the implement to another. More complex equipment, such as silage choppers and snowblowers, have drives with outputs in more than one direction; the gearbox in a wind turbine converts the slow, high-torque rotation of the turbine into much faster rotation of the electrical generator. These are more complicated than the PTO gearboxes in farm equipment, they weigh several tons and contain three stages to achieve an overall gear ratio from 40:1 to over 100:1, depending on the size of the turbine. The first stage of the gearbox is a planetary gear, for compactness, to distribute the enormous torque of the turbine over more teeth of the low-speed shaft.
Durability of these gearboxes has been a serious problem for a long time. Regardless of where they are used, these simple transmissions all share an important feature: the gear ratio cannot be changed during use, it is fixed at the time. For transmission types that overcome this issue, see Continuously variable transmission known as CVT. Many applications require the availability of multiple gear ratios; this is to ease the starting and stopping of a mechanical system, though another important need is that of maintaining good fuel efficiency. The need for a transmission in an automobile is a consequence of the characteristics of the internal combustion engine. Eng
A beam axle, rigid axle or solid axle is a dependent suspension design, in which a set of wheels is connected laterally by a single beam or shaft. Beam axles were once used at the rear wheels of a vehicle, but they have been used as front axles in rear-wheel-drive vehicles. In most automobiles, beam axles have been replaced by front and rear independent suspensions. With a beam axle the camber angle between the wheels is the same no matter where it is in the travel of the suspension. A beam axle's fore & aft location is constrained by either: trailing arms, semi-trailing arms, radius rods, or leaf springs; the lateral location can be constrained by a Panhard rod, a Scott Russell linkage or a Watt's linkage, or some other arrangement, most by the leaf springs. Shock absorbers and either leaf springs, coil springs, or air bags are used to control vertical movement; the Twist-beam rear suspension is a similar suspension design, however its beam axle is able to twist thereby functioning as an anti-roll bar to control the roll motion of the body and is considered to be a semi-independent suspension design.
A live axle is a type of beam axle in which the shaft transmits power to the wheels. While used in vehicles with Hotchkiss drive, this suspension system can be used with other types of power transmission; the principal advantage of the beam axle is its simplicity. This simplicity makes it space-efficient and cheap to manufacture, they are nearly universally used in heavy-duty trucks. Most light and medium duty pickup trucks, SUVs, vans use a beam axle, at least in the rear. Beam axles have an important advantage for off-road applications, as they provide better vehicle articulation and durability in a high load environment; the drawbacks are that it does not allow each wheel to move independently in response to bumps, the mass of the beam is part of the unsprung weight of the vehicle, which can further reduce ride quality. The cornering ability is worse than other suspension designs because the wheels have zero camber angle gain during body roll. Front beam axle suspension is unusually sensitive to any lack of concentricity in the hub and wheel assembly which can cause a side-to-side oscillation of the steering at certain speeds.
This is addressed on some vehicles with steering dampers although removal and careful refitting of the front wheels cures the problem. Axle Twist-beam rear suspension
Aluminium or aluminum is a chemical element with symbol Al and atomic number 13. It is a silvery-white, soft and ductile metal in the boron group. By mass, aluminium makes up about 8% of the Earth's crust; the chief ore of aluminium is bauxite. Aluminium metal is so chemically reactive that native specimens are rare and limited to extreme reducing environments. Instead, it is found combined in over 270 different minerals. Aluminium is remarkable for its low density and its ability to resist corrosion through the phenomenon of passivation. Aluminium and its alloys are vital to the aerospace industry and important in transportation and building industries, such as building facades and window frames; the oxides and sulfates are the most useful compounds of aluminium. Despite its prevalence in the environment, no known form of life uses aluminium salts metabolically, but aluminium is well tolerated by plants and animals; because of these salts' abundance, the potential for a biological role for them is of continuing interest, studies continue.
Of aluminium isotopes, only 27Al is stable. This is consistent with aluminium having an odd atomic number, it is the only aluminium isotope that has existed on Earth in its current form since the creation of the planet. Nearly all the element on Earth is present as this isotope, which makes aluminium a mononuclidic element and means that its standard atomic weight equates to that of the isotope; the standard atomic weight of aluminium is low in comparison with many other metals, which has consequences for the element's properties. All other isotopes of aluminium are radioactive; the most stable of these is 26Al and therefore could not have survived since the formation of the planet. However, 26Al is produced from argon in the atmosphere by spallation caused by cosmic ray protons; the ratio of 26Al to 10Be has been used for radiodating of geological processes over 105 to 106 year time scales, in particular transport, sediment storage, burial times, erosion. Most meteorite scientists believe that the energy released by the decay of 26Al was responsible for the melting and differentiation of some asteroids after their formation 4.55 billion years ago.
The remaining isotopes of aluminium, with mass numbers ranging from 21 to 43, all have half-lives well under an hour. Three metastable states are known, all with half-lives under a minute. An aluminium atom has 13 electrons, arranged in an electron configuration of 3s23p1, with three electrons beyond a stable noble gas configuration. Accordingly, the combined first three ionization energies of aluminium are far lower than the fourth ionization energy alone. Aluminium can easily surrender its three outermost electrons in many chemical reactions; the electronegativity of aluminium is 1.61. A free aluminium atom has a radius of 143 pm. With the three outermost electrons removed, the radius shrinks to 39 pm for a 4-coordinated atom or 53.5 pm for a 6-coordinated atom. At standard temperature and pressure, aluminium atoms form a face-centered cubic crystal system bound by metallic bonding provided by atoms' outermost electrons; this crystal system is shared by some other metals, such as copper. Aluminium metal, when in quantity, is shiny and resembles silver because it preferentially absorbs far ultraviolet radiation while reflecting all visible light so it does not impart any color to reflected light, unlike the reflectance spectra of copper and gold.
Another important characteristic of aluminium is its low density, 2.70 g/cm3. Aluminium is a soft, lightweight and malleable with appearance ranging from silvery to dull gray, depending on the surface roughness, it is nonmagnetic and does not ignite. A fresh film of aluminium serves as a good reflector of visible light and an excellent reflector of medium and far infrared radiation; the yield strength of pure aluminium is 7–11 MPa, while aluminium alloys have yield strengths ranging from 200 MPa to 600 MPa. Aluminium has stiffness of steel, it is machined, cast and extruded. Aluminium atoms are arranged in a face-centered cubic structure. Aluminium has a stacking-fault energy of 200 mJ/m2. Aluminium is a good thermal and electrical conductor, having 59% the conductivity of copper, both thermal and electrical, while having only 30% of copper's density. Aluminium is capable of superconductivity, with a superconducting critical temperature of 1.2 kelvin and a critical magnetic field of about 100 gauss.
Aluminium is the most common material for the fabrication of superconducting qubits. Aluminium's corrosion resistance can be excellent due to a thin surface layer of aluminium oxide that forms when the bare metal is exposed to air preventing further oxidation, in a process termed passivation; the strongest aluminium alloys are less corrosion resistant due to galvanic reactions with alloyed copper. This corrosion resistance is reduced by aqueous salts in the presence of dissimilar metals. In acidic solutions, aluminium reacts with water to form hydrogen, in alkaline ones to form aluminates—protective passivation under these conditions is negligible; because it is corroded by dissolved chlorides, such as common sodium chloride, household plumbing is never made from aluminium. However, because
Wire wheels, wire-spoked wheels, tension-spoked wheels, or "suspension" wheels are wheels whose rims connect to their hubs by wire spokes. Although these wires are stiffer than a typical wire rope, they function mechanically the same as tensioned flexible wires, keeping the rim true while supporting applied loads; the term suspension wheel should not be confused with vehicle suspension. Wire wheels are still used on many motorcycles, they were invented by aeronautical engineer George Cayley in 1808. Although Cayley first proposed wire wheels, he did not apply for a patent; the first patent for wire wheels was issued to Theodore Jones of London, England on October 11, 1826. Eugène Meyer of Paris, France was the first person to receive, in 1869, a patent for wire wheels on bicycles. Bicycle wheels were not strong enough for cars until the development of tangentially spoked wheels, they became well established in the bicycle and motor tricycle world but were not common on cars until around 1907. This was encouraged by the Rudge-Whitworth patented detachable and interchangeable wheels designed by John Pugh.
These wheels owed their resistance to braking and accelerative stresses to their two inner rows of tangential spokes. An outer row of radial spokes gave; these wheels were dished so that steering pivot pins might lie as near as possible to the center-line of the tires. Their second feature was that they were detachable being mounted on splined false hubs. A process of assembling wire wheels is described as wheelbuilding. From the earliest days automobiles used either wire wheels or heavy wooden or pressed steel spoked artillery type; the development of the quick detachable hubs of either Rudge-Whitworth or Riley design did much to popularise wire wheels and incidentally led to the fitting of "spare wheels". After their wooden spoked artillery wheels proved inadequate many US manufacturers paid John Pugh of Rudge-Whitworth royalties to manufacture wire wheels using his patents. Artillery wheels fell out of favour in the late 1920s and the development of the cheaper pressed steel wheels by Joseph Sankey replaced wire wheels wherever the premium price of wire wheels was not justified by their weight saving.
A sample of cars first riding on wire wheels Before 1960, sports/racing cars had Rudge-Whitworth wire wheels center-locking equipped with splined hubs and a quick-release "knockoff" locking cap that could be unscrewed by striking a wing of the nut with a special alloy mallet or "knockoff hammer". Some jurisdictions, including West Germany, prohibited eared hubcaps; some manufacturers preferred to hold the wheel on the splined hub by capping with a single conventional unwinged nut requiring a special large spanner. In the 1960s lighter cast alloy wheels became usual—at first with splined hubs and knock-off caps—and now predominate. New versions of wire wheels are still made but with standard hub bolt patterns covered by a center cap to fit without adapters. At one time, motorcycles used wire wheels built up from separate components, except for dirtbikes, they are now used for their retro appearance; the first commercially successful use of wired wheels was on bicycles. They were introduced early on in the development of the bicycle, following soon after the adoption of solid rubber tires.
This development marked a major improvement over the older wooden wheels, both in terms of weight and comfort. In England, the engineer William Stanley developed the steel-wired spider wheel in 1849, an improvement over the cumbersome wooden spoked wheels fitted to the tricycles that his employer was making. Bicycle manufacturers build millions of wheels annually, using the common crossed-spoke patterns whose crossings of adjacent spokes are governed by the number of spokes in the wheel. Wheelbuilders of racing teams and in good bicycle shops build wheels to other patterns such as two-cross, one-cross, or no-cross. Many of these patterns have been used for more than 100 years, it is claimed that crossed patterns have more strength and stability while irregular patterns are art forms and have little structural merit. In the 1980s, cast wheels with 5 or 6 rigid spokes began to appear in the Olympic Games and professional racing: these have advantages in specialized applications, such as time trials, but wire-spoked wheels are used for most purposes.
The reaction to a radial load of a well-tensioned wire spoked wheel, such as by a rider sitting on a bicycle, is that the wheel flattens near the ground contact area. The rest of the wheel remains circular; the tension of all the spokes does not increase significantly. Instead, only the spokes directly under the hub decrease their tension; the issue of how best to describe this situation is debated. Some authors conclude from this that the hub "stands" on those spokes below it that experience a reduction in tension though the spokes below the hub exert no upward force on the hub and can be replaced by chains without much changing the physics of the wheel. Other authors conclude that the hub "hangs" from those spokes above it that exert an upward force on the hub, that have higher tension than the spokes below the hub, which pull down on the hub. Despite being composed of thin and flexible spokes, wire wheels are radially stiff and provide little suspension compliance compared to high-pressure bicycle tires.
Astounding.org.uk, an analysis of the deflection of wire wheels. Duke.edu, an analysis of the deflection of wire wheels
A carburetor or carburettor is a device that mixes air and fuel for internal combustion engines in the proper air–fuel ratio for combustion. It is sometimes colloquially shortened to carby in Australia. To carburate or carburet means to mix the air and fuel or to equip with a carburetor for that purpose. Carburetors have been supplanted in the automotive and, to a lesser extent, aviation industries by fuel injection, they are still common on small engines for lawn mowers and other equipment. The word carburetor comes from the French carbure meaning "carbide". Carburer means to combine with carbon. In fuel chemistry, the term has the more specific meaning of increasing the carbon content of a fluid by mixing it with a volatile hydrocarbon; the first carburetor was invented by Samuel Morey in 1826. The first person to patent a carburetor for use in a petroleum engine was Siegfried Marcus with his 6 July 1872 patent for a device which mixes fuel with air. A carburetor was among the early patents by Karl Benz as he developed internal combustion engines and their components.
Early carburetors were of the surface type, in which air is combined with fuel by passing over the surface of gasoline. In 1885, Wilhelm Maybach and Gottlieb Daimler developed a float carburetor based on the atomizer nozzle; the Daimler-Maybach carburetor was copied extensively. British courts rejected the Daimler company's claim of priority in favor of Edward Butler's 1884 spray carburetor used on his Petrol Cycle. Hungarian engineers János Csonka and Donát Bánki patented a carburetor for a stationary engine in 1893. Frederick William Lanchester of Birmingham, experimented with the wick carburetor in cars. In 1896, Frederick and his brother built a gasoline-driven car in England, a single cylinder 5 hp internal combustion engine with chain drive. Unhappy with the car's performance and power, they re-designed the engine the following year using two horizontally-opposed cylinders and a newly designed wick carburetor. Carburetors were the common method of fuel delivery for most US-made gasoline engines until the late 1980s, when fuel injection became the preferred method.
This change was dictated by the requirements of catalytic converters and not due to an inherent inefficiency of carburation. A catalytic converter requires that there be more precise control over the fuel / air mixture in order to control the amount of oxygen remaining in the exhaust gases. In the U. S. market, the last cars using carburetors were: 1990: Oldsmobile Custom Cruiser, Buick Estate Wagon, Cadillac Brougham, Honda Prelude, Subaru Justy 1991: Ford Crown Victoria Police Interceptor with the 5.8 L V8 engine. 1991: Jeep Grand Wagoneer with the AMC 360 cu in V8 engine. 1993: Mazda B2200 1994: IsuzuIn Australia, some cars continued to use carburetors well into the 1990s. Low-cost commercial vans and 4WDs in Australia continued with carburetors into the 2000s, the last being the Mitsubishi Express van in 2003. Elsewhere, certain Lada cars used carburetors until 2006. Many motorcycles still use carburetors for simplicity's sake, since a carburetor does not require an electrical system to function.
Carburetors are still found in small engines and in older or specialized automobiles, such as those designed for stock car racing, though NASCAR's 2011 Sprint Cup season was the last one with carbureted engines. In Europe, carburetor-engined cars were being phased out by the end of the 1980s in favor of fuel injection, the established type of engine on more expensive vehicles including luxury and sports models. EEC legislation required all vehicles sold and produced in member countries to have a catalytic converter after December 1992; this legislation had been in the pipeline for some time, with many cars becoming available with catalytic converters or fuel injection from around 1990. However, some versions of the Peugeot 106 were sold with carburettor engines from its launch in 1991, as were versions of the Renault Clio and Nissan Primera and all versions of Ford Fiesta range except the XR2i when it was launched in 1989. Luxury car manufacturer Mercedes-Benz had been producing mechanically fuel-injected cars since the early 1950s, while the first mainstream family car to feature fuel injection was the Volkswagen Golf GTI in 1976.
Ford's first fuel-injected car was the Ford Capri RS 2600 in 1970. General Motors launched its first fuel-injected car in 1957 as an option available for the first generation Corvette. Saab switched to fuel injection across its whole range from 1982; the carburetor works on Bernoulli's principle: the faster air moves, the lower its static pressure, higher the dynamic pressure is. The throttle linkage does not directly control the flow of liquid fuel. Instead, it actuates carburetor mechanisms which meter the flow of air being carried into the engine; the speed of this flow, therefore its pressure, determines the amount of fuel drawn into the airstream. When carburetors are used in aircraft with piston engines, special designs and features are needed to prevent fuel starvation during inverted flight. Engines used an early form of fuel injection known as a pressure carburetor. Most production carbureted engines, as opposed to fuel-injected, h
World War I
World War I known as the First World War or the Great War, was a global war originating in Europe that lasted from 28 July 1914 to 11 November 1918. Contemporaneously described as "the war to end all wars", it led to the mobilisation of more than 70 million military personnel, including 60 million Europeans, making it one of the largest wars in history, it is one of the deadliest conflicts in history, with an estimated nine million combatants and seven million civilian deaths as a direct result of the war, while resulting genocides and the 1918 influenza pandemic caused another 50 to 100 million deaths worldwide. On 28 June 1914, Gavrilo Princip, a Bosnian Serb Yugoslav nationalist, assassinated the Austro-Hungarian heir Archduke Franz Ferdinand in Sarajevo, leading to the July Crisis. In response, on 23 July Austria-Hungary issued an ultimatum to Serbia. Serbia's reply failed to satisfy the Austrians, the two moved to a war footing. A network of interlocking alliances enlarged the crisis from a bilateral issue in the Balkans to one involving most of Europe.
By July 1914, the great powers of Europe were divided into two coalitions: the Triple Entente—consisting of France and Britain—and the Triple Alliance of Germany, Austria-Hungary and Italy. Russia felt it necessary to back Serbia and, after Austria-Hungary shelled the Serbian capital of Belgrade on the 28th, partial mobilisation was approved. General Russian mobilisation was announced on the evening of 30 July; when Russia failed to comply, Germany declared war on 1 August in support of Austria-Hungary, with Austria-Hungary following suit on 6th. German strategy for a war on two fronts against France and Russia was to concentrate the bulk of its army in the West to defeat France within four weeks shift forces to the East before Russia could mobilise. On 2 August, Germany demanded free passage through Belgium, an essential element in achieving a quick victory over France; when this was refused, German forces invaded Belgium on 3 August and declared war on France the same day. On 12 August and France declared war on Austria-Hungary.
In November 1914, the Ottoman Empire entered the war on the side of the Alliance, opening fronts in the Caucasus and the Sinai Peninsula. The war was fought in and drew upon each power's colonial empire as well, spreading the conflict to Africa and across the globe; the Entente and its allies would become known as the Allied Powers, while the grouping of Austria-Hungary and their allies would become known as the Central Powers. The German advance into France was halted at the Battle of the Marne and by the end of 1914, the Western Front settled into a battle of attrition, marked by a long series of trench lines that changed little until 1917. In 1915, Italy opened a front in the Alps. Bulgaria joined the Central Powers in 1915 and Greece joined the Allies in 1917, expanding the war in the Balkans; the United States remained neutral, although by doing nothing to prevent the Allies from procuring American supplies whilst the Allied blockade prevented the Germans from doing the same the U. S. became an important supplier of war material to the Allies.
After the sinking of American merchant ships by German submarines, the revelation that the Germans were trying to incite Mexico to make war on the United States, the U. S. declared war on Germany on 6 April 1917. Trained American forces would not begin arriving at the front in large numbers until mid-1918, but the American Expeditionary Force would reach some two million troops. Though Serbia was defeated in 1915, Romania joined the Allied Powers in 1916 only to be defeated in 1917, none of the great powers were knocked out of the war until 1918; the 1917 February Revolution in Russia replaced the Tsarist autocracy with the Provisional Government, but continuing discontent at the cost of the war led to the October Revolution, the creation of the Soviet Socialist Republic, the signing of the Treaty of Brest-Litovsk by the new government in March 1918, ending Russia's involvement in the war. This allowed the transfer of large numbers of German troops from the East to the Western Front, resulting in the German March 1918 Offensive.
This offensive was successful, but the Allies rallied and drove the Germans back in their Hundred Days Offensive. Bulgaria was the first Central Power to sign an armistice—the Armistice of Salonica on 29 September 1918. On 30 October, the Ottoman Empire capitulated. On 4 November, the Austro-Hungarian empire agreed to the Armistice of Villa Giusti after being decisively defeated by Italy in the Battle of Vittorio Veneto. With its allies defeated, revolution at home, the military no longer willing to fight, Kaiser Wilhelm abdicated on 9 November and Germany signed an armistice on 11 November 1918. World War I was a significant turning point in the political, cultural and social climate of the world; the war and its immediate aftermath sparked numerous uprisings. The Big Four (Britain, the United States, It