Radar is a detection system that uses radio waves to determine the range, angle, or velocity of objects. It can be used to detect aircraft, spacecraft, guided missiles, motor vehicles, weather formations, terrain. A radar system consists of a transmitter producing electromagnetic waves in the radio or microwaves domain, a transmitting antenna, a receiving antenna and a receiver and processor to determine properties of the object. Radio waves from the transmitter reflect off the object and return to the receiver, giving information about the object's location and speed. Radar was developed secretly for military use by several nations in the period before and during World War II. A key development was the cavity magnetron in the UK, which allowed the creation of small systems with sub-meter resolution; the term RADAR was coined in 1940 by the United States Navy as an acronym for RAdio Detection And Ranging The term radar has since entered English and other languages as a common noun, losing all capitalization.
The modern uses of radar are diverse, including air and terrestrial traffic control, radar astronomy, air-defense systems, antimissile systems, marine radars to locate landmarks and other ships, aircraft anticollision systems, ocean surveillance systems, outer space surveillance and rendezvous systems, meteorological precipitation monitoring and flight control systems, guided missile target locating systems, ground-penetrating radar for geological observations, range-controlled radar for public health surveillance. High tech radar systems are associated with digital signal processing, machine learning and are capable of extracting useful information from high noise levels. Radar is a key technology that the self-driving systems are designed to use, along with sonar and other sensors. Other systems similar to radar make use of other parts of the electromagnetic spectrum. One example is "lidar". With the emergence of driverless vehicles, Radar is expected to assist the automated platform to monitor its environment, thus preventing unwanted incidents.
As early as 1886, German physicist Heinrich Hertz showed that radio waves could be reflected from solid objects. In 1895, Alexander Popov, a physics instructor at the Imperial Russian Navy school in Kronstadt, developed an apparatus using a coherer tube for detecting distant lightning strikes; the next year, he added a spark-gap transmitter. In 1897, while testing this equipment for communicating between two ships in the Baltic Sea, he took note of an interference beat caused by the passage of a third vessel. In his report, Popov wrote that this phenomenon might be used for detecting objects, but he did nothing more with this observation; the German inventor Christian Hülsmeyer was the first to use radio waves to detect "the presence of distant metallic objects". In 1904, he demonstrated the feasibility of detecting a ship in dense fog, but not its distance from the transmitter, he obtained a patent for his detection device in April 1904 and a patent for a related amendment for estimating the distance to the ship.
He got a British patent on September 23, 1904 for a full radar system, that he called a telemobiloscope. It operated on a 50 cm wavelength and the pulsed radar signal was created via a spark-gap, his system used the classic antenna setup of horn antenna with parabolic reflector and was presented to German military officials in practical tests in Cologne and Rotterdam harbour but was rejected. In 1915, Robert Watson-Watt used radio technology to provide advance warning to airmen and during the 1920s went on to lead the U. K. research establishment to make many advances using radio techniques, including the probing of the ionosphere and the detection of lightning at long distances. Through his lightning experiments, Watson-Watt became an expert on the use of radio direction finding before turning his inquiry to shortwave transmission. Requiring a suitable receiver for such studies, he told the "new boy" Arnold Frederic Wilkins to conduct an extensive review of available shortwave units. Wilkins would select a General Post Office model after noting its manual's description of a "fading" effect when aircraft flew overhead.
Across the Atlantic in 1922, after placing a transmitter and receiver on opposite sides of the Potomac River, U. S. Navy researchers A. Hoyt Taylor and Leo C. Young discovered that ships passing through the beam path caused the received signal to fade in and out. Taylor submitted a report, suggesting that this phenomenon might be used to detect the presence of ships in low visibility, but the Navy did not continue the work. Eight years Lawrence A. Hyland at the Naval Research Laboratory observed similar fading effects from passing aircraft. Before the Second World War, researchers in the United Kingdom, Germany, Japan, the Netherlands, the Soviet Union, the United States, independently and in great secrecy, developed technologies that led to the modern version of radar. Australia, New Zealand, South Africa followed prewar Great Britain's radar development, Hungary generated its radar technology during the war. In France in 1934, following systematic studies on the split-anode magnetron, the research branch of the Compagnie Générale de Télégraphie Sans Fil headed by Maurice Ponte with Henri Gutton, Sylvain Berline and M. Hugon, began developing an obstacle-locatin
An early-warning radar is any radar system used for the long-range detection of its targets, i.e. allowing defences to be alerted as early as possible before the intruder reaches its target, giving the air defences the maximum time in which to operate. This contrasts with systems used for tracking or gun laying, which tend to offer shorter ranges but offer much higher accuracy. EW radars tend to share a number of design features. For instance, EW radar operates at lower frequencies, thus longer wavelengths, than other types; this reduces their interaction with rain and snow in the air, therefore improves their performance in the long-range role where their coverage area will include precipitation. This has the side-effect of lowering their optical resolution, but this is not important in this role. EW radars use much lower pulse repetition frequency to maximize their range, at the cost of signal strength, offset this with long pulse widths, which increases the signal at the cost of lowering range resolution.
The canonical EW radar is the British Chain Home system, which entered full-time service in 1938. It used a low pulse repetition of 25 pps and powerful transmissions reaching 1 MW in late-war upgrades; the German Freya and US CXAM and SCR-270 were similar. Post-war models moved to the microwave range in ever-increasingly powerful models that reached the 50 MW range by the 1960s. Since improvements in receiver electronics has reduced the amount of signal needed to produce an accurate image, modern examples the transmitted power is much less. EW radars are highly susceptible to radar jamming and include advanced frequency hopping systems to reduce this problem; the first early-warning radars were the British Chain Home, the German Freya, the US CXAM and SCR-270, the Soviet Union RUS-2. By modern standards these were quite short range about 100 to 150 miles; this "short" distance is a side effect of radio propagation at the long wavelengths being used at the time, which were limited to line-of-sight.
Although techniques for long-range propagation were known and used for shortwave radio, the ability to process the complex return signal was not possible at the time. To counter the threat of Soviet bombers flying over the Arctic, the U. S. and Canada developed the DEW Line. Other examples have since been built with better performance. An alternative early warning design was the McGill Fence, which provided "line breaking" indication across the middle of Canada, with no provision to identify the target's exact location or direction of travel. Starting in the 1950s, a number of over-the-horizon radars were developed that extended detection ranges by bouncing the signal off the ionosphere. Today the early warning role has been supplanted to a large degree by airborne early warning platforms. By placing the radar on an aircraft, the line-of-sight to the horizon is extended; this allows the radar to use high-frequency signals, offering high resolution, while still offering long range. A major exception to this rule are radars intended to warn of ballistic missile attacks, like BMEWS, as the high-altitude exo-atmospheric trajectory of these weapons allows them to be seen at great ranges from ground-based radars.
Chain Home Chain Home Low SCR-270 AN/CPS-1 CXAM radar Freya radar Pinetree Line McGill Fence Distant Early Warning Line Duga radar BMEWS AMES Type 80 AMES Type 84 AMES Type 85 ROTOR Dnestr radar Dnepr radar Daryal radar Linesman/Mediator AWACS Daryal radar Dnestr radar Don-2N radar Duga radar Dunay radar GIRAFFE EL/M-2080 Green Pine EL/M-2090 Erieye Jindalee Long Range Discrimination Radar North Warning System PAVE PAWS Red Color Sea-based X-band Radar Voronezh radar
In antenna theory, a phased array means an electronically scanned array, a computer-controlled array of antennas which creates a beam of radio waves that can be electronically steered to point in different directions without moving the antennas. In an array antenna, the radio frequency current from the transmitter is fed to the individual antennas with the correct phase relationship so that the radio waves from the separate antennas add together to increase the radiation in a desired direction, while cancelling to suppress radiation in undesired directions. In a phased array, the power from the transmitter is fed to the antennas through devices called phase shifters, controlled by a computer system, which can alter the phase electronically, thus steering the beam of radio waves to a different direction. Since the array must consist of many small antennas to achieve high gain, phased arrays are practical at the high frequency end of the radio spectrum, in the UHF and microwave bands, in which the antenna elements are conveniently small.
Phased arrays were invented for use in military radar systems, to scan the radar beam across the sky to detect planes and missiles. These phased array radar systems are now used, phased arrays are spreading to civilian applications; the phased array principle is used in acoustics, phased arrays of acoustic transducers are used in medical ultrasound imaging scanners and gas prospecting, military sonar systems. The term "phased array" is used to a lesser extent for unsteered array antennas in which the phase of the feed power and thus the radiation pattern of the antenna array is fixed. For example, AM broadcast radio antennas consisting of multiple mast radiators fed so as to create a specific radiation pattern are called "phased arrays". A passive phased array or passive electronically scanned array is a phased array in which the antenna elements are connected to a single transmitter and/or receiver, as shown in the animation at top. PESAs are the most common type of phased array. An active phased array or active electronically scanned array is a phased array in which each antenna element has its own transmitter/receiver unit, all controlled by the computer.
Active arrays are a more advanced, second-generation phased-array technology which are used in military applications. A conformal antenna is a phased array in which the individual antennas, instead of being arranged in a flat plane, are mounted on a curved surface; the phase shifters compensate for the different path lengths of the waves due to the antenna elements' varying position on the surface, allowing the array to radiate a plane wave. Conformal antennas are used in aircraft and missiles, to integrate the antenna into the curving surface of the aircraft to reduce aerodynamic drag. Phased array transmission was shown in 1905 by Nobel laureate Karl Ferdinand Braun who demonstrated enhanced transmission of radio waves in one direction. During World War II, Nobel laureate Luis Alvarez used phased array transmission in a steerable radar system for "ground-controlled approach", a system to aid in the landing of aircraft. At the same time, the GEMA in Germany built the Mammut 1, it was adapted for radio astronomy leading to Nobel Prizes for Physics for Antony Hewish and Martin Ryle after several large phased arrays were developed at the University of Cambridge.
This design is used for radar, is generalized in interferometric radio antennas. In 2004, Caltech researchers demonstrated the first integrated silicon-based phased array receiver at 24GHz with 8 elements; this was followed by their demonstration of a CMOS 24GHz phased array transmitter in 2005 and a integrated 77GHz phased array transceiver with integrated antennas in 2006 by the Caltech team. In 2007, DARPA researchers announced a 16 element phased array radar antenna, integrated with all the necessary circuits on a single silicon chip and operated at 30–50 GHz; the relative amplitudes of—and constructive and destructive interference effects among—the signals radiated by the individual antennas determine the effective radiation pattern of the array. A phased array may be used to point a fixed radiation pattern, or to scan in azimuth or elevation. Simultaneous electrical scanning in both azimuth and elevation was first demonstrated in a phased array antenna at Hughes Aircraft Company, California in 1957.
In broadcast engineering, phased arrays are used by many AM broadcast radio stations to enhance signal strength and therefore coverage in the city of license, while minimizing interference to other areas. Due to the differences between daytime and nighttime ionospheric propagation at mediumwave frequencies, it is common for AM broadcast stations to change between day and night radiation patterns by switching the phase and power levels supplied to the individual antenna elements daily at sunrise and sunset. For shortwave broadcasts many stations use arrays of horizontal dipoles. A common arrangement uses 16 dipoles in a 4×4 array; this is in front of a wire grid reflector. The phasing is switchable to allow Beam steering in azimuth and sometimes elevation. More modest phased array longwire antenna systems may be employed by private radio enthusiasts to receive longwave and shortwave radio broadcasts from great distances. On VHF, phased arrays are used extensively for FM broadcasting; these increase the antenna gain, magnifying the emitted RF energy toward the horizon, which in turn increases a station's broadcast range
RX12874 known as the Passive Detection System and by its nickname "Winkle", was a radar detector system used as part of the Royal Air Force's Linesman/Mediator radar network until the early 1980s. Winkle passed out of service along with the rest of the Linesman system as the IUKADGE network replaced it. Winkle was developed in the late 1950s to counter the carcinotron, a radar jammer so effective that it was believed it would render all long-range radars useless. Winkle used a network of stations to listen for carcinotron broadcasts, combined the information from them to track the jammer aircraft as as a radar could; the system was based on AMES Type 85 radars. Both were used as receivers. Information from HSAs and the Type 85s was combined in a correlator that used triangulation and time-of-flight information to determine the location of the jammer-carrying aircraft. Once the location was determined, it was manually input into the interception controller's displays as if it were a normal radar return, distinguished only by its small circle icon instead of a single dot.
Operators could decrease the Type 85 receiver sensitivity while the radar passed that location, so that the jamming did not obscure the display at nearby angles. Combined with identification friend or foe signals, this allowed a fighter aircraft's signal to remain visible and interceptions could proceed as normal. In 1950, engineers at the French company CSF introduced the carcinotron, a microwave-producing vacuum tube that could be tuned across a wide range of frequencies by changing a single input voltage. By continually sweeping through the frequencies of known radars, it would overpower the radar's own reflections, blind them, its wide bandwidth meant that a single carcinotron could be used to send jamming signals against any radar it was to meet, the rapid tuning meant it could do so against multiple radars at the same time, or sweep through all potential frequencies to produce barrage jamming. The carcinotron was revealed publicly in November 1953; the Admiralty Signals and Radar Establishment purchased one and fit it to a Handley Page Hastings named Catherine, testing it against the latest AMES Type 80 radar late that year.
As they feared, it rendered the radar display unreadable, filled with noise that hid any real targets. Useful jamming was accomplished when the aircraft was under the radar horizon, in which case other aircraft had to be 20 miles to the sides before they were visible outside the jamming signal; the system was so powerful. The Type 80 was a key part of the ROTOR system, a comprehensive radar and control network covering the entire British Isles; the Catherine tests suggested that the system would be rendered impotent before it was fully installed. The Royal Aircraft Establishment began developing their own carcinotrons for the V Bomber force under the name Indigo Bracket, while solutions to the jamming problem for the RAF's radars were studied; the first consideration was that the carcinotron provided a weak signal, on the order of 5 kW. When used in barrage mode, this was diluted to 5 to 10 W per MHz of bandwidth. Due to the radar equation, at long range this was still much stronger than the reflection of the multi-megawatt signal from the radar itself.
As the jamming aircraft approached the station, there was some point where the radar began to overpower the jammer, the "self-screening" or "burn-through" point. A powerful transmitter would increase the range where this occurred. Further improvement could be gained by focusing the beam to put as much power into the reflected signal as possible; the Royal Radar Establishment began development of such a system with Metropolitan-Vickers under the name "Blue Riband". It was assumed. Through the use of twelve 4.5 MW klystron transmitters broadcast through an enormous 75 by 50 foot antenna system, the Blue Riband produced 11.4 W per MHz of reflected signal at 200 miles, thereby overpowering the assumed threat. To force the jammer to spread out its signal across a wide band, the radar randomly changed frequencies with every pulse, across a 500 MHz bandwidth. Through this period there had been an ongoing debate about the usefulness of air defences; the introduction of the hydrogen bomb meant a single aircraft could destroy any target, the higher speeds and altitudes of bomber aircraft meant the bombs could be dropped from further away.
By 1954, the Chief of the Air Staff had concluded that close defence was useless, began plans to remove anti-aircraft artillery from the defence. By December, planners believed the only practical role for air defence was to protect the V-force while it was launching. In keeping with this role, over the next few years the number of radar stations and fighters continued to be reduced as the protected area contracted around the Midlands; the 1957 Defence White Paper shifted priorities from manned bombers to missiles. The only way to defend against a missile attack was deterrence, so it was vital that the V-force survive; this meant that any attack, whether by aircraft or missiles, would require the V-force to launch immediately. By the end of 1957, the idea of any defence of the deterrent force had been abandoned
A backward wave oscillator called carcinotron or backward wave tube, is a vacuum tube, used to generate microwaves up to the terahertz range. Belonging to the traveling-wave tube family, it is an oscillator with a wide electronic tuning range. An electron gun generates an electron beam, it sustains the oscillations by propagating a traveling wave backwards against the beam. The generated electromagnetic wave power has its group velocity directed oppositely to the direction of motion of the electrons; the output power is coupled out near the electron gun. It has two main subtypes, the M-type, the most powerful, the O-type; the output power of the O-type is in the range of 1 mW at 1000 GHz to 50 mW at 200 GHz. Carcinotrons are used as stable microwave sources. Due to the good quality wavefront they produce, they find use as illuminators in terahertz imaging; the backward wave oscillators were demonstrated in 1951, M-type by Bernard Epsztein and O-type by Rudolf Kompfner. The M-type BWO is a voltage-controlled non-resonant extrapolation of magnetron interaction.
Both types are tunable over a wide range of frequencies by varying the accelerating voltage. They can be swept through the band fast enough to be appearing to radiate over all the band at once, which makes them suitable for effective radar jamming tuning into the radar frequency. Carcinotrons allowed airborne radar jammers to be effective. However, frequency-agile radars can hop frequencies fast enough to force the jammer to use barrage jamming, diluting its output power over a wide band and impairing its efficiency. Carcinotrons are used in research and military applications. For example, the Czechoslovak Kopac passive sensor and Ramona passive sensor air defense detection systems employed carcinotrons in their receiver systems. All travelling-wave tubes operate in the same general fashion, differ in details of their construction; the concept is dependent on a steady stream of electrons from an electron gun that travel down the center of the tube. Surrounding the electron beam is some sort of radio frequency source signal.
As the electrons travel down the tube, they interact with the RF signal. The electrons are repelled from negative areas; this causes the electrons to bunch up as they are repelled or attracted along the length of the tube, a process known as velocity modulation. This process makes the electron beam take on the same general structure as the original signal; the result is a signal in the electron beam, an amplified version of the original RF signal. As the electrons are moving, they induce a magnetic field in any nearby conductor as illustrated in the concept diagram; this allows the now-amplified signal to be extracted. In systems like the magnetron or klystron, this is accomplished with another resonant cavity. In the helical designs, this process occurs along the entire length of the tube, reinforcing the original signal in the helical conductor; the "problem" with traditional designs is that they have narrow bandwidths. The BWO is built in a fashion similar to the helical TWT. However, instead of the RF signal propagating in the same direction as the electron beam, the original signal travels at right angles to the beam.
This is accomplished by drilling a hole through a rectangular waveguide and shooting the beam through the hole. The waveguide goes through two right angle turns, forming a C-shape and crossing the beam again; this basic pattern is repeated along the length of the tube so the waveguide passes across the beam several times, forming a series of S-shapes. The original RF signal enters from what would be the far end of the TWT, where the energy would be extracted; the effect of the signal on the passing beam causes the same velocity modulation effect, but because of the direction of the RF signal and specifics of the waveguide, this modulation travels backward along the beam, instead of forward. This propagation, the slow-wave, reaches the next hole in the folded waveguide just as the same phase of the RF signal does; this causes amplification just like the traditional TWT. The difference in the two systems is that in the TWT the speed of propagation in the helix has to be similar to that of the electrons in the beam.
This is not the case in the BWO. The waveguide places strict limits on the bandwidth of the signal and sets its propagation speed as a basic function of its construction, but the speed of the signal induced into the electron beam is relative to the speed of the electrons; that means the frequency of the output signal can be changed by changing the speed of the electrons, accomplished by changing the voltage of the electron gun. The device was given the name "carcinotron" because it was like cancer to existing radar systems. By changing the supply voltage, the device could produce any required frequency across a band, much larger than any existing microwave amplifier could match - the cavity magnetron and klystron worked
Royal Air Force
The Royal Air Force is the United Kingdom's aerial warfare force. Formed towards the end of the First World War on 1 April 1918, it is the oldest independent air force in the world. Following victory over the Central Powers in 1918 the RAF emerged as, at the time, the largest air force in the world. Since its formation, the RAF has taken a significant role in British military history. In particular, it played a large part in the Second World War where it fought its most famous campaign, the Battle of Britain; the RAF's mission is to support the objectives of the British Ministry of Defence, which are to "provide the capabilities needed to ensure the security and defence of the United Kingdom and overseas territories, including against terrorism. The RAF describes its mission statement as "... an agile and capable Air Force that, person for person, is second to none, that makes a decisive air power contribution in support of the UK Defence Mission". The mission statement is supported by the RAF's definition of air power.
Air power is defined as "the ability to project power from the air and space to influence the behaviour of people or the course of events". Today the Royal Air Force maintains an operational fleet of various types of aircraft, described by the RAF as being "leading-edge" in terms of technology; this consists of fixed-wing aircraft, including: fighter and strike aircraft, airborne early warning and control aircraft, ISTAR and SIGINT aircraft, aerial refueling aircraft and strategic and tactical transport aircraft. The majority of the RAF's rotary-wing aircraft form part of the tri-service Joint Helicopter Command in support of ground forces. Most of the RAF's aircraft and personnel are based in the UK, with many others serving on operations or at long-established overseas bases. Although the RAF is the principal British air power arm, the Royal Navy's Fleet Air Arm and the British Army's Army Air Corps deliver air power, integrated into the maritime and land environments. While the British were not the first to make use of heavier-than-air military aircraft, the RAF is the world's oldest independent air force: that is, the first air force to become independent of army or navy control.
Following publication of the "Smuts report" prepared by Jan Smuts the RAF was founded on 1 April 1918, with headquarters located in the former Hotel Cecil, during the First World War, by the amalgamation of the Royal Flying Corps and the Royal Naval Air Service. At that time it was the largest air force in the world. After the war, the service was drastically cut and its inter-war years were quiet, with the RAF taking responsibility for the control of Iraq and executing a number of minor actions in other parts of the British Empire; the RAF's naval aviation branch, the Fleet Air Arm, was founded in 1924 but handed over to Admiralty control on 24 May 1939. The RAF developed the doctrine of strategic bombing which led to the construction of long-range bombers and became its main bombing strategy in the Second World War; the RAF underwent rapid expansion prior to and during the Second World War. Under the British Commonwealth Air Training Plan of December 1939, the air forces of British Commonwealth countries trained and formed "Article XV squadrons" for service with RAF formations.
Many individual personnel from these countries, exiles from occupied Europe served with RAF squadrons. By the end of the war the Royal Canadian Air Force had contributed more than 30 squadrons to serve in RAF formations approximately a quarter of Bomber Command's personnel were Canadian. Additionally, the Royal Australian Air Force represented around nine percent of all RAF personnel who served in the European and Mediterranean theatres. In the Battle of Britain in 1940, the RAF defended the skies over Britain against the numerically superior German Luftwaffe. In what is the most prolonged and complicated air campaign in history, the Battle of Britain contributed to the delay and subsequent indefinite postponement of Hitler's plans for an invasion of the United Kingdom. In the House of Commons on 20 August, prompted by the ongoing efforts of the RAF, Prime Minister Winston Churchill eloquently made a speech to the nation, where he said "Never in the field of human conflict was so much owed by so many to so few".
The largest RAF effort during the war was the strategic bombing campaign against Germany by Bomber Command. While RAF bombing of Germany began immediately upon the outbreak of war, under the leadership of Air Chief Marshal Harris, these attacks became devastating from 1942 onward as new technology and greater numbers of superior aircraft became available; the RAF adopted night-time area bombing on German cities such as Hamburg and Dresden, developed precision bombing techniques for specific operations, such as the "Dambusters" raid by No. 617 Squadron, or the Amiens prison raid known as Operation Jericho. Following victory in the Second World War, the RAF underwent significant re-organisation, as technological advances in air warfare saw the arrival of jet fighters and bombers. During the early stages of the Cold War, one of the first major operations undertaken by the Royal Air Force was in 1948 and the Berlin Airlift, codenamed Operation Plainfire. Between 26 June and the lifting of the Russian blockade of the city on 2 May, the RAF provided 17% of the total supplies delivered du
World War II
World War II known as the Second World War, was a global war that lasted from 1939 to 1945. The vast majority of the world's countries—including all the great powers—eventually formed two opposing military alliances: the Allies and the Axis. A state of total war emerged, directly involving more than 100 million people from over 30 countries; the major participants threw their entire economic and scientific capabilities behind the war effort, blurring the distinction between civilian and military resources. World War II was the deadliest conflict in human history, marked by 50 to 85 million fatalities, most of whom were civilians in the Soviet Union and China, it included massacres, the genocide of the Holocaust, strategic bombing, premeditated death from starvation and disease, the only use of nuclear weapons in war. Japan, which aimed to dominate Asia and the Pacific, was at war with China by 1937, though neither side had declared war on the other. World War II is said to have begun on 1 September 1939, with the invasion of Poland by Germany and subsequent declarations of war on Germany by France and the United Kingdom.
From late 1939 to early 1941, in a series of campaigns and treaties, Germany conquered or controlled much of continental Europe, formed the Axis alliance with Italy and Japan. Under the Molotov–Ribbentrop Pact of August 1939, Germany and the Soviet Union partitioned and annexed territories of their European neighbours, Finland and the Baltic states. Following the onset of campaigns in North Africa and East Africa, the fall of France in mid 1940, the war continued between the European Axis powers and the British Empire. War in the Balkans, the aerial Battle of Britain, the Blitz, the long Battle of the Atlantic followed. On 22 June 1941, the European Axis powers launched an invasion of the Soviet Union, opening the largest land theatre of war in history; this Eastern Front trapped most crucially the German Wehrmacht, into a war of attrition. In December 1941, Japan launched a surprise attack on the United States as well as European colonies in the Pacific. Following an immediate U. S. declaration of war against Japan, supported by one from Great Britain, the European Axis powers declared war on the U.
S. in solidarity with their Japanese ally. Rapid Japanese conquests over much of the Western Pacific ensued, perceived by many in Asia as liberation from Western dominance and resulting in the support of several armies from defeated territories; the Axis advance in the Pacific halted in 1942. Key setbacks in 1943, which included a series of German defeats on the Eastern Front, the Allied invasions of Sicily and Italy, Allied victories in the Pacific, cost the Axis its initiative and forced it into strategic retreat on all fronts. In 1944, the Western Allies invaded German-occupied France, while the Soviet Union regained its territorial losses and turned toward Germany and its allies. During 1944 and 1945 the Japanese suffered major reverses in mainland Asia in Central China, South China and Burma, while the Allies crippled the Japanese Navy and captured key Western Pacific islands; the war in Europe concluded with an invasion of Germany by the Western Allies and the Soviet Union, culminating in the capture of Berlin by Soviet troops, the suicide of Adolf Hitler and the German unconditional surrender on 8 May 1945.
Following the Potsdam Declaration by the Allies on 26 July 1945 and the refusal of Japan to surrender under its terms, the United States dropped atomic bombs on the Japanese cities of Hiroshima and Nagasaki on 6 and 9 August respectively. With an invasion of the Japanese archipelago imminent, the possibility of additional atomic bombings, the Soviet entry into the war against Japan and its invasion of Manchuria, Japan announced its intention to surrender on 15 August 1945, cementing total victory in Asia for the Allies. Tribunals were set up by fiat by the Allies and war crimes trials were conducted in the wake of the war both against the Germans and the Japanese. World War II changed the political social structure of the globe; the United Nations was established to foster international co-operation and prevent future conflicts. The Soviet Union and United States emerged as rival superpowers, setting the stage for the nearly half-century long Cold War. In the wake of European devastation, the influence of its great powers waned, triggering the decolonisation of Africa and Asia.
Most countries whose industries had been damaged moved towards economic expansion. Political integration in Europe, emerged as an effort to end pre-war enmities and create a common identity; the start of the war in Europe is held to be 1 September 1939, beginning with the German invasion of Poland. The dates for the beginning of war in the Pacific include the start of the Second Sino-Japanese War on 7 July 1937, or the Japanese invasion of Manchuria on 19 September 1931. Others follow the British historian A. J. P. Taylor, who held that the Sino-Japanese War and war in Europe and its colonies occurred and the two wars merged in 1941; this article uses the conventional dating. Other starting dates sometimes used for World War II include the Italian invasion of Abyssinia on 3 October 1935; the British historian Antony Beevor views the beginning of World War II as the Battles of Khalkhin Gol fought between Japan and the fo