The United Nations was created to foster international cooperation and prevent future conflicts, with the victorious great powers—China, France, the Soviet Union, the UK, and the U.S.—becoming the permanent members of its security council.

[8] The British historian Antony Beevor views the beginning of World War II as the Battles of Khalkhin Gol fought between Japan and the forces of Mongolia and the Soviet Union from May to September 1939.

[15] World War I had radically altered the political European map with the defeat of the Central Powers—including Austria-Hungary, Germany, Bulgaria, and the Ottoman Empire—and the 1917 Bolshevik seizure of power in Russia, which led to the founding of the Soviet Union.

Meanwhile, the victorious Allies of World War I, such as France, Belgium, Italy, Romania, and Greece, gained territory, and new nation-states were created out of the dissolution of the Austro-Hungarian, Ottoman, and Russian Empires.

Under the treaty, Germany lost around 13 percent of its home territory and all its overseas possessions, while German annexation of other states was prohibited, reparations were imposed, and limits were placed on the size and capability of the country's armed forces.

Following Hindenburg's death in 1934, Hitler proclaimed himself Führer of Germany and abolished democracy, espousing a radical, racially motivated revision of the world order, and soon began a massive rearmament campaign.

The situation was aggravated in early 1935 when the Territory of the Saar Basin was legally reunited with Germany, and Hitler repudiated the Treaty of Versailles, accelerated his rearmament programme, and introduced conscription.

In 1931, an increasingly militaristic Empire of Japan, which had long sought influence in China[28] as the first step of what its government saw as the country's right to rule Asia, staged the Mukden incident as a pretext to invade Manchuria and establish the puppet state of Manchukuo.

[38] In July 1937, Japan captured the former Chinese imperial capital of Peking after instigating the Marco Polo Bridge incident, which culminated in the Japanese campaign to invade all of China.

This policy would prove difficult to maintain in light of the Japanese defeat at Khalkin Gol in 1939, the ongoing Second Sino-Japanese War[49] and ally Nazi Germany pursuing neutrality with the Soviets.

Immediately afterwards, Hitler ordered the attack to proceed on 26 August, but upon hearing that the United Kingdom had concluded a formal mutual assistance pact with Poland and that Italy would maintain neutrality, he decided to delay it.

[64] The Poles refused to comply with the German demands, and on the night of 30–31 August in a confrontational meeting with the British ambassador Nevile Henderson, Ribbentrop declared that Germany considered its claims rejected.

The German strategic bombing offensive intensified with night attacks on London and other cities in the Blitz, but largely ended in May 1941[108] after failing to significantly disrupt the British war effort.

[119] To assist Italy and prevent Britain from gaining a foothold, Germany prepared to invade the Balkans, which would threaten Romanian oil fields and strike against British dominance of the Mediterranean.

Hitler's objectives were to eliminate the Soviet Union as a military power, exterminate Communism, generate Lebensraum ("living space")[138] by dispossessing the native population,[139] and guarantee access to the strategic resources needed to defeat Germany's remaining rivals.

[143] 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[144] prompted the United Kingdom to reconsider its grand strategy.

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 invaded and occupied northern Indochina in September 1940.

The British argued that military operations should target peripheral areas to wear out German strength, leading to increasing demoralisation, and bolstering resistance forces; Germany itself would be subject to a heavy bombing campaign.

[214] Despite considerable losses, in early 1942 Germany and its allies stopped a major Soviet offensive in central and southern Russia, keeping most territorial gains they had achieved during the previous year.

[242] This decision was partially affected by the Western Allies' invasion of Sicily launched on 9 July, which, combined with previous Italian failures, resulted in the ousting and arrest of Mussolini later that month.

[254] The former conference determined the post-war return of Japanese territory[255] and the military planning for the Burma campaign,[256] while the latter included agreement that the Western Allies would invade Europe in 1944 and that the Soviet Union would declare war on Japan within three months of Germany's defeat.

[280] By this point, the communist-led Partisans under Marshal Josip Broz Tito, who had led an increasingly successful guerrilla campaign against the occupation since 1941, controlled much of the territory of Yugoslavia and engaged in delaying efforts against German forces further south.

[301] Meanwhile, the United States Army Air Forces launched a massive firebombing campaign of strategic cities in Japan in an effort to destroy Japanese war industry and civilian morale.

Between the two bombings, the Soviets, pursuant to the Yalta agreement, declared war on Japan, invaded Japanese-held Manchuria and quickly defeated the Kwantung Army, which was the largest Japanese fighting force.

[359] The Soviet Union, despite enormous human and material losses, also experienced rapid increase in production in the immediate post-war era,[360] having seized and transferred most of Germany's industrial plants and exacted war reparations from its satellite states.

[392] The Soviet Union was responsible for the Katyn massacre of 22,000 Polish officers,[393] and the imprisonment or execution of hundreds of thousands of political prisoners by the NKVD secret police, along with mass civilian deportations to Siberia, in the Baltic states and eastern Poland annexed by the Red Army.

They also murdered an additional 4 million others who were deemed "unworthy of life" (including the disabled and mentally ill, Soviet prisoners of war, Romani, homosexuals, Freemasons, and Jehovah's Witnesses) as part of a program of deliberate extermination, in effect becoming a "genocidal state".

[417] Unlike in the West, the Nazi racial policy encouraged extreme brutality against what it considered to be the "inferior people" of Slavic descent; most German advances were thus followed by mass atrocities and war crimes.

[425] The United States produced about two-thirds of all munitions used by the Allies in World War II, including warships, transports, warplanes, artillery, tanks, trucks, and ammunition.

Although aeronautical warfare had relatively little success at the start of the war, actions at Taranto, Pearl Harbor, and the Coral Sea established the carrier as the dominant capital ship (in place of the battleship).