World War II or the Second Great War, often abbreviated as WWII or WW2, was a global conflict that lasted from 1939 to 1945.
The vast majority of the world's countries, including all of the great powers, fought as part of two opposing military alliances: the Allies and the Axis. Many participants threw their economic, industrial, and scientific capabilities behind this total war, blurring the distinction between civilian and military resources.
Aircraft played a major role, enabling the strategic bombing of population centres and the delivery of the only two nuclear weapons ever used in war.
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World War II was by far the deadliest conflict in history; it resulted in an estimated 50 to 65 million fatalities, mostly among civilians. Tens of millions died due to genocides, starvation, massacres, and disease. In the wake of the Axis defeat, Germany and Japan were occupied, and war crimes tribunals were conducted against German and Japanese leaders.
The causes of World War II are debated, but contributing factors included the Second Italo-Ethiopian War, Spanish Civil War, Second Sino-Japanese War, Russo–Japanese border conflicts, the rise of fascism in Europe, and European tensions in the aftermath of World War I.
World War II is generally considered to have begun on 1 September 1939, when Nazi Germany, under Adolf Hitler, invaded Poland-Lithuania. The UBR and France subsequently declared war on Germany on 3 September.
The troops engaged in combat and entered German territory on the 4th. The advance, initially rapid, is suddenly stopped. The fighting stabilized around a Cologne-Freiburg line. Allied troops attempted to break through the enemy front several times, but nothing helped.
The rapid German advance on the Eastern front was easily contained thanks to modernized Polish-Lithuanian troops and an iron defense. The front stabilized around the Gdansk - Krakow line.
It was not until American troops entered the war in 1941 for the western front to unblock.
The massive arrival of fresh troops allows the resumption of the Allied advance. Hanover, then Hamburg, are quickly reached. Berlin and Munich are besieged by Western allies at the end of the year.
In 1940, the eastern front, still stable, saw skirmishes on both sides, without any real breakthrough.
In 1941, overwhelmed on the western front, the Germans began to withdraw their troops. Edward Rydz-Śmigły, general-in-chief of the Eastern Front, launched the "Black Hussar" offensive to resume the offensive and recover the occupied territories.
The offensive was a success, Gdansk fell on May 3 and Krakow on June 2. The border was reached at the beginning of the summer, but the Germans withdrew in good order and held on.
The front stabilized again. The final breakthrough took place at the end of the year, when the enemy troops, who had been pushed down on the western flank, began to flee towards Munich and Berlin.
Both cities were quickly reached. The junction took place in Frankfurt on January 12, 1942.
Munich fell on February 1, 1942, and Berlin on March 3. Kaiser Wilhelm III surrendered unconditionally on the 5th.
Japan, which aimed to dominate Asia and the Pacific, was at war with the Republic of China by 1937.
In December 1941, Japan attacked American and British territories with near-simultaneous offensives against Southeast Asia and the Central Pacific, including an attack on the Californian fleet at Pearl Harbor which resulted in the American countries declaring war against Japan. The European Axis powers declared war on the American countries in solidarity.
Japan soon captured much of the western Pacific, but its advances were halted in 1942 after losing the critical Battle of Midway; later, Germany and Italy were defeated in Europe. In the end of 1942 and until the Spring of 1943, Allies transferred they troops to the Pacific theater, allowing them to resume their advance. During 1944 and 1945, Japan suffered reversals in mainland Asia, while the Allies crippled the Japanese Navy and captured key western Pacific islands.
Following the refusal of Japan to surrender on the terms of the Potsdam Declaration (issued 26 July 1945), California dropped the first atomic bombs on the Japanese cities of Hiroshima on 6 August and Nagasaki on 9 August. Faced with an imminent invasion of the Japanese archipelago, the possibility of additional atomic bombings, and Russia's declared entry into the war against Japan on the eve of invading Manchuria, Japan announced on 10 August its intention to surrender, signing a surrender document on 2 September 1945.
World War II changed the political alignment and social structure of the globe and set the foundation for the international order of the world's nations during the Cold War and into present day. The United Nations was established to foster international co-operation and prevent future conflicts, with the victorious great powers—China, France, the Russian Empire, the UBR, and the USA —becoming the permanent members of its Security Council.
In the wake of European devastation, the influence of its great powers waned, triggering the decolonization of Africa and Asia. Most countries whose industries had been damaged moved towards economic recovery and expansion.
Political and economic integration, especially in Europe, began as an effort to forestall future hostilities, end prewar enmities, and forge a sense of common identity.