World War II, or the Second World War[1] (often abbreviated WWII or WW2), was a global military conflict lasting from 1939 to 1945 which involved most of the world's nations, including all great powers, organised into two opposing military alliances: the Allies and the Axis. It was the most widespread war in history, with more than 100 million military personnel mobilised. In a state of "total war," the major participants placed their entire economic, industrial, and scientific capabilities at the service of the war effort, erasing the distinction between civilian and military resources. Marked by significant action against civilians, including The Holocaust and the first use of nuclear weapons in warfare, it was the deadliest conflict in human history,[2] with over seventy million casualties.
The start of the war is generally held to be September 1, 1939, with the invasion of Poland by Nazi Germany and subsequent declarations of war on Germany by France and most of the countries of the British Empire and Commonwealth. Many countries were already at war by this date, such as Ethiopia and Italy in the Second Italo-Abyssinian War and China and Japan in the Second Sino-Japanese War.[3] Many that were not initially involved joined the war later in response to events such as the German invasion of the Soviet Union and the Japanese attacks on the U.S. Pacific Fleet at Pearl Harbor and on British overseas colonies, which triggered declarations of war on Japan by the United States, the British Commonwealth,[4] and the Netherlands.[5]
In 1945 the war ended in an Allied victory and a changed world. While the United Nations was established to foster international cooperation and prevent future conflicts, the Soviet Union and the United States emerged as two rival superpowers, setting the stage for the Cold War which lasted for the next 46 years. Meanwhile, the acceptance of the principle of self-determination accelerated decolonization movements in Asia and Africa, while Western Europe began moving toward economic recovery and increased political integration.

The start of the war is generally held to be September 1, 1939 beginning with the German invasion of Poland; Britain and France declared war on Germany two days later. Other dates for the beginning of war include the Japanese invasion of Manchuria on September 13, 1931;[6] the start of the Second Sino-Japanese War on July 7, 1937;[7][8] or one of several other events.
Others follow A. J. P. Taylor, who held that there was a simultaneous Sino-Japanese War in East Asia, and a Second European War in Europe and her colonies. The two wars merged in 1941, becoming a single global conflict, at which point the war continued until 1945. This article uses the conventional dating.[9]
The exact date of the war's end is not universally agreed upon. It has been suggested that the war ended at the armistice of August 14, 1945 (V-J Day), rather than the formal surrender of Japan (September 2, 1945); in some European histories, it ended on V-E Day (May 8, 1945). The Treaty of Peace with Japan was not signed until 1951.

World War I radically altered the diplomatic and political situations in Eurasia and Africa with the defeat of the Central Powers, including Austria-Hungary, Germany, and the Ottoman Empire; and the Bolshevik seizure of power in Russia in 1917. Meanwhile the success of the Allied Entente powers including the United Kingdom, France, the United States, Italy, Yugoslavia, and Romania and the creation of new states from the collapse of Austria-Hungary and the Russian Empire resulted in a major shift in the balance of power in Europe.
In the aftermath of World War I major unrest in Europe rose, especially irredentist and revanchist nationalism and class conflict. Irredentism and revanchism was strong in Germany which was forced to accept significant territorial, colonial, and financial losses as part of the Treaty of Versailles. In the interwar period, domestic civil conflict occurred in Germany involving nationalists and reactionaries versus communists and moderate democratic political parties. A similar scenario was in Italy. Although Italy as an Entente ally made some territorial gains, Italian nationalists were angered that the terms of the Treaty of London upon which Italy had agreed to wage war on the Central Powers, were not fulfilled with the peace settlement. From 1922 to 1925, the Italian Fascist movement led by Benito Mussolini seized power in Italy with a nationalist, totalitarian, and class collaborationist agenda that abolished representative democracy, repressed political forces supporting class conflict or liberalism, and pursued an aggressive foreign policy aimed at forcefully forging Italy as a world power. Fascism became internationally popular amongst people disillusioned with democratic government, liberalism, and class conflict.
In Germany, the Nazi Party led by Adolf Hitler pursued establishing such a fascist government in Germany. With the onset of the Great Depression, Nazi support rose and in 1933, Hitler was appointed Chancellor of Germany, and in the aftermath of the Reichstag fire, Hitler created a totalitarian single-party state led by the Nazis.

The Kuomintang (KMT) party in China launched a unification campaign against regional warlords and nominally unified China in the mid-1920s, but was soon embroiled in a civil war against its former Chinese communist allies.[11] In 1931, an increasingly militaristic Japanese Empire, which had long sought influence in China[12] as the first step of its right to rule Asia, used the Mukden Incident as justification to invade Manchuria and established the puppet state of Manchukuo.[13] Too weak to resist Japan, China appealed to the League of Nations for help. Japan withdrew from the League of Nations after being condemned for its incursion into Manchuria. The two nations then fought several minor conflicts, in Shanghai, Rehe and Hebei, until signing the Tanggu Truce in 1933. Thereafter, Chinese volunteer forces continued the resistance to Japanese aggression in Manchuria, and Chahar and Suiyuan.[14]

Adolf Hitler, after an unsuccessful attempt to overthrow the German government in 1923, became the Chancellor of Germany in 1933. He abolished democracy, espousing a radical, racially-motivated revision of the world order, and soon began a massive rearmament campaign.[15] Meanwhile, France, to secure its alliance, allowed Italy a free hand in Ethiopia, which Italy desired as a colonial possession. The situation was aggravated in early 1935 when the Saarland was legally reunited with Germany and Hitler repudiated the Treaty of Versailles, speeding up his rearmament programme and introducing conscription.[16]
Hoping to contain Germany, the United Kingdom, France and Italy formed the Stresa Front. The Soviet Union, concerned due to Germany's goals of capturing vast areas of eastern Europe, wrote a treaty of mutual assistance with France. Before taking effect though, the Franco-Soviet pact was required to go through the bureaucracy of the League of Nations, which rendered it essentially toothless.[17][18]
In June 1935, the United Kingdom made an independent naval agreement with Germany, easing prior restrictions. The United States, concerned with events in Europe and Asia, passed the Neutrality Act in August.[19] In October, Italy invaded Ethiopia, with Germany the only major European nation supporting the invasion. Italy then revoked objections to Germany's goal of absorbing Austria.[20]
Hitler defied the Versailles and Locarno treaties by remilitarizing the Rhineland in March 1936. He received little response from other European powers.[21] When the Spanish Civil War broke out in July, Hitler and Mussolini supported fascist Generalissimo Francisco Franco's nationalist forces in his civil war against the Soviet-supported Spanish Republic. Both sides used the conflict to test new weapons and methods of warfare,[22] and the nationalists won the war in early 1939. Mounting tensions led to several efforts to strengthen or consolidate power. In October 1936, Germany and Italy formed the Rome-Berlin Axis. A month later, Germany and Japan signed the Anti-Comintern Pact, which Italy would join in the following year. In China, after the Xian Incident the Kuomintang and communist forces agreed on a ceasefire in order to present a united front to oppose Japan.[23]

The Second Italo–Abyssinian War was a brief colonial war that started in October 1935 and ended in May 1936. The war was fought between the armed forces of the Kingdom of Italy (Regno d'Italia) and the armed forces of the Ethiopian Empire (also known as Abyssinia). The war resulted in the military occupation of Ethiopia and its annexation into the newly created colony of Italian East Africa (Africa Orientale Italiana, or AOI); in addition, it exposed the weakness of the League of Nations as a force to preserve peace. Both Italy and Ethiopia were member nations, but the League did nothing when the former clearly violated the League's own Article X.[24]

In July 1937, Japan captured the former imperial capital of Beiping after instigating the Marco Polo Bridge Incident, which culminated in the Japanese campaign to invade all of China.[25] The Soviets quickly signed a non-aggression pact with China to lend materiel support, effectively ending China's prior cooperation with Germany. Generalissimo Chiang Kai-shek deployed his best army to defend Shanghai, but after 3 month of fighting Shanghai fell. The Japanese continue to push the Chinese forces back, capturing the capital Nanjing in December 1937 and committed the Nanking Massacre.
In June 1938, Chinese forces stalled the Japanese advance by flooding the Yellow River; although this manoeuvre bought time for the Chinese to prepare their defences at Wuhan, the city was taken by October.[26] However, Japanese military victories did not bring about the collapse of Chinese resistance that Japan had hoped to achieve, instead the Chinese government relocated to Chongqing to continue their resistance.[27]
Japanese invasion of the USSR and Mongolia
See also: Hokushin-ron, Nanshin-ron, and Soviet-Japanese Border War (1939)


Soviet troops during the Battle of Khalkin Gol.
On July 29, 1938, the Japanese invaded the USSR and were checked at the Battle of Lake Khasan. Although the battle was a Soviet victory, the Japanese dismissed it as an inconclusive draw, and on May 11, 1939 decided to move the Japanese-Mongolian border up to the Khalkin Gol River by force. Stalin replaced the former Soviet commander with Georgy Zhukov on Semyon Timoshenko's advice. Zhukov, along with reinforcements sent from Moscow, checked the Japanese assault on Mongolia and handed the Japanese Kwangtung Army their first major defeat.[28][29]

hese clashes convinced the Japanese government that they should focus on conciliating the Soviet government to avoid interference in the war against China and instead turn their military attention southward, towards the US and European holdings in the Pacific. They also prevented the sacking of experienced Soviet military leaders such as Zhukov, who would later play a vital role in the defence of Moscow.[30]
European occupations and agreements
Further information: Anschluss, German occupation of Czechoslovakia, and Molotov-Ribbentrop Pact


Czechs watch German troops enter Prague after Czechoslovakia capitulates, March 15, 1939.
In Europe, Germany and Italy were becoming bolder. In March 1938, Germany annexed Austria, again provoking little response from other European powers.[31] Encouraged, Hitler began pressing German claims on the Sudetenland, an area of Czechoslovakia with a predominantly ethnic German population; France and Britain conceded this territory to him, against the wishes of the Czechoslovak government, in exchange for a promise of no further territorial demands.[32] Soon after that, however, Germany and Italy forced Czechoslovakia to cede additional territory to Hungary and Poland.[33] In March 1939, Germany invaded the remainder of Czechoslovakia and subsequently split it into the German Protectorate of Bohemia and Moravia and the pro-German client state, the Slovak Republic.[34]
Alarmed, and with Hitler making further demands on Danzig, France and Britain guaranteed their support for Polish independence; when Italy conquered Albania in April 1939, the same guarantee was extended to Romania and Greece.[35] Shortly after the Franco-British pledge to Poland, Germany and Italy formalised their own alliance with the Pact of Steel.[36] In August 1939, Germany and the Soviet Union signed the Molotov–Ribbentrop Pact, a non-aggression pact.[37] This treaty included a secret protocol placing western Poland and Lithuania in the German sphere of influence while placing eastern Poland, Finland, Estonia, Latvia and the Romanian province of Bessarabia in the Soviet sphere of influence.[38]

On September 1, 1939, Germany and Slovakia — a client state in 1939 — attacked Poland. France, Britain, and the countries of the Commonwealth declared war on Germany but provided little military support to Poland other than a small French attack into the Saarland.[39] On September 17, 1939, after signing an armistice with Japan, the Soviets launched their own invasion of Poland.[40] By early October, Poland was divided among Germany, the Soviet Union, Lithuania and Slovakia, although Poland never officially surrendered and continued the fight outside its borders.[41] At the same time as the battle in Poland, Japan launched its first attack against Changsha, a strategically important Chinese city, but was repulsed by late September.[42]


Common parade of German Wehrmacht and Soviet Red Army on September 23, 1939 in Brest, Eastern Poland at the end of the Invasion of Poland. At centre is Major General Heinz Guderian and at right is Brigadier Semyon Krivoshein.
Following the invasion of Poland and a German-Soviet treaty governing Lithuania, the Soviet Union forced the Baltic countries to allow it to station Soviet troops in their countries under pacts of "mutual assistance."[43][44][45] Finland rejected territorial demands and was invaded by the Soviet Union in November 1939.[46] The resulting conflict ended in March 1940 with Finnish concessions.[47] France and the United Kingdom, treating the Soviet attack on Finland as tantamount to entering the war on the side of the Germans, responded to the Soviet invasion by supporting the USSR's expulsion from the League of Nations.[45] In June 1940, the Soviet Armed Forces invaded and occupied the neutral Baltic States.[44]


German troops in Paris after the fall of France
In Western Europe, British troops deployed to the Continent, but in a phase nicknamed the Phoney War by the British and "Sitzkrieg" (sitting war) by the Germans, neither side launched major operations against the other until April 1940.[48] The Soviet Union and Germany entered a trade pact in February of 1940, pursuant to which the Soviets received German military and industrial equipment in exchange for supplying raw materials to Germany to help circumvent a British blockade.[49] In April, Germany invaded Denmark and Norway to secure shipments of iron ore from Sweden, which the Allies would try to disrupt.[50] Denmark immediately capitulated, and despite Allied support, Norway was conquered within two months.[51] British discontent over the Norwegian campaign led to the replacement of Prime Minister Neville Chamberlain by Winston Churchill on May 10, 1940.[52]

Axis advances


German and other Axis conquests (in blue) in Europe, during World War II
On that same day, Germany invaded France, Belgium, the Netherlands, and Luxembourg.[53] The Netherlands and Belgium were overrun using blitzkrieg tactics in a few days and weeks, respectively.[54] The French fortified Maginot Line was circumvented by a flanking movement through the thickly wooded Ardennes region,[53] mistakenly perceived by French planners as an impenetrable natural barrier against armoured vehicles.[55] British troops were forced to evacuate the continent at Dunkirk, abandoning their heavy equipment by the end of the month. On June 10, Italy invaded, declaring war on both France and the United Kingdom;[56] twelve days later France surrendered and was soon divided into German and Italian occupation zones,[57] and an unoccupied rump state under the Vichy Regime. On July 14, the British attacked the French fleet in Algeria to prevent its possible seizure by Germany.[58]
With France neutralised, Germany began an air superiority campaign over Britain (the Battle of Britain) to prepare for an invasion.[59] The campaign failed, and the invasion plans were cancelled by September. Using newly captured French ports, the German Navy enjoyed success against an over-extended Royal Navy, using U-boats against British shipping in the Atlantic.[60] Italy began operations in the Mediterranean, initiating a siege of Malta in June, conquering British Somaliland in August, and making an incursion into British-held Egypt in September 1940. Japan increased its blockade of China in September by seizing several bases in the northern part of the now-isolated French Indochina.[61]


The Battle of Britain ended the German advance in Western Europe
Throughout this period, the neutral United States took measures to assist China and the Western Allies. In November 1939, the American Neutrality Act was amended to allow 'Cash and carry' purchases by the Allies.[62] In 1940, following the German capture of Paris, the size of the United States Navy was significantly increased and, after the Japanese incursion into Indochina, the United States embargoed iron, steel and mechanical parts against Japan.[63] In September, the United States further agreed to a trade of American destroyers for British bases.[64] Still, a large majority of the American public continued to oppose any direct military intervention into the conflict well into 1941.[65]
At the end of September 1940, the Tripartite Pact united Japan, Italy, and Germany to formalize the Axis Powers.[66] The Tripartite Pact stipulated that any country, with the exception of the Soviet Union, not in the war which attacked any Axis Power would be forced to go to war against all three.[67] During this time, the United States continued to support the United Kingdom and China by introducing the Lend-Lease policy authorizing the provision of war materiel and other items[68] and creating a security zone spanning roughly half of the Atlantic Ocean where the United States Navy protected British convoys.[69] As a result, Germany and the United States found themselves engaged in sustained naval warfare in the North and Central Atlantic by October 1941, even though the United States remained officially neutral.[70][71]

The Axis expanded in November 1940 when Hungary, Slovakia, and Romania joined the Tripartite Pact.[72] These countries participated in the subsequent invasion of the USSR, with Romania making the largest contribution to recapture territory ceded to the USSR and pursue its leader Ion Antonescu's desire to combat communism.[73] In October 1940, Italy invaded Greece but within days was repulsed and pushed back into Albania, where a stalemate soon occurred.[74] In December 1940, British Commonwealth forces began counter-offensives against Italian forces in Egypt and Italian East Africa.[75] By early 1941, with Italian forces having been pushed back into Libya by the Commonwealth, Churchill ordered a dispatch of troops from Africa to bolster the Greeks.[76] The Italian Navy also suffered significant defeats, with the Royal Navy putting three Italian battleships out of commission by carrier attack at Taranto, and neutralising several more warships at Cape Matapan.[77]


German paratroopers invading Crete
The Germans soon intervened to assist Italy. Hitler sent German forces to Libya in February, and by the end of March they had launched an offensive against the diminished Commonwealth forces.[78] In under a month, Commonwealth forces were pushed back into Egypt with the exception of the besieged port of Tobruk.[79] The Commonwealth attempted to dislodge Axis forces in May and again in June, but failed on both occasions.[80] In early April, following Bulgaria's signing of the Tripartite Pact, the Germans intervened in the Balkans by invading Greece and Yugoslavia following a coup; here too they made rapid progress, eventually forcing the Allies to evacuate after Germany conquered the Greek island of Crete by the end of May.[81]
The Allies did have some successes during this time. In the Middle East, Commonwealth forces first quashed a coup in Iraq which had been supported by German aircraft from bases within Vichy-controlled Syria,[82] then, with the assistance of the Free French, invaded Syria and Lebanon to prevent further such occurrences.[83] In the Atlantic, the British scored a much-needed public morale boost by sinking the German flagship Bismarck.[84] Perhaps most importantly, during the Battle of Britain the Royal Air Force had successfully resisted the Luftwaffe's assault, and on May 11, 1941, Hitler called off the bombing campaign.[85]
In Asia, despite several offensives by both sides, the war between China and Japan was stalemated by 1940. In order to increase pressure on China by blocking supply routes, and to better position Japanese forces in the event of a war with the Western powers, Japan had seized military control of southern Indochina[86] In August of that year, Chinese communists launched an offensive in Central China; in retaliation, Japan instituted harsh measures (the Three Alls Policy) in occupied areas to reduce human and material resources for the communists.[87] Continued antipathy between Chinese communist and nationalist forces culminated in armed clashes in January 1941, effectively ending their co-operation.[88] With the situation in Europe and Asia relatively stable, Germany, Japan, and the Soviet Union made preparations. With the Soviets wary of mounting tensions with Germany and the Japanese planning to take advantage of the European War by seizing resource-rich European possessions in Southeast Asia, the two powers signed the Soviet–Japanese Neutrality Pact in April 1941.[89] By contrast, the Germans were steadily making preparations for an attack on the Soviet Union, amassing forces on the Soviet border.[90]

The war becomes global


A German soldier inspecting the remains of destroyed Soviet forces in the Białystok–Minsk pocket.
On June 22, 1941, Germany, along with other European Axis members and Finland, invaded the Soviet Union in Operation Barbarossa. The primary targets of this surprise offensive[91] were the Baltic region, Moscow, and Ukraine, with an ultimate goal of ending the 1941 campaign near the Arkhangelsk-Astrakhan line, connecting the Caspian and White Seas. Hitler's objectives were to eliminate the Soviet Union as a military power, exterminate Communism, generate Lebensraum ("living space")[92] by dispossessing the native population[93] and guarantee access to the strategic resources needed to defeat Germany's remaining rivals.[94]
Although the Red Army was preparing for strategic counter-offensives before the war,[95] Barbarossa forced the Soviet supreme command to adopt a strategic defence. During the summer, the Axis made significant gains into Soviet territory, inflicting immense losses in both personnel and materiel. By the middle of August, however, the German Army High Command decided to suspend the offensive of a considerably depleted Army Group Centre, and to divert the Second Panzer Group to reinforce troops advancing toward central Ukraine and Leningrad.[96] The Kiev offensive was overwhelmingly successful, resulting in encirclement and elimination of four Soviet armies, and made further advance into Crimea and industrially developed Eastern Ukraine (the First Battle of Kharkov) possible.[97]


German infantry and armoured vehicles battle the Soviet defenders on the streets of Kharkov in 1941.
The diversion of three quarters of the Axis troops and the majority of their air forces from France and the central Mediterranean to the Eastern Front[98][99] prompted the United Kingdom to reconsider its grand strategy.[100] In July, the UK and the Soviet Union formed a military alliance against Germany[101] and jointly invaded Iran shortly afterwards to secure the Persian Corridor and Iran's oilfields.[102] In August, the United Kingdom and the United States jointly issued the Atlantic Charter.[103]
By October, when Axis operational objectives in Ukraine and the Baltic region were achieved, with only the sieges of Leningrad[104] and Sevastopol continuing,[105] a major offensive against Moscow had been renewed. After two months of fierce battles, the German army almost reached the outer suburbs of Moscow, where the exhausted troops[106] were forced to suspend their offensive.[107] Large territorial gains were made by Axis forces, but their campaign had failed to achieve its main objectives: two key cities remained in Soviet hands, the Soviet capability to resist was not broken, and the Soviet Union retained a considerable part of its military potential. The blitzkrieg phase of the war in Europe had ended.[108]
By early December, freshly mobilised reserves[109] allowed the Soviets to achieve numerical parity with Axis troops.[110] This, as well as intelligence data that established a minimal number of Soviet troops in the East sufficient to prevent any attack by the Japanese Kwantung Army,[111] allowed the Soviets to begin a massive counter-offensive that started on December 5 along a 1,000 kilometres (620 mi) front and pushed German troops 100–250 kilometres (62–160 mi) west.[112]