Causes of World War II explained to kids

World War II was a global conflict that lasted from 1939 to 1945. It involved most of the world's nations, including all of the great powers, eventually forming two opposing military alliances: the Allies and the Axis. It was the most widespread war in history, with more than 100 million people serving in military units. 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 mass deaths of civilians, including the Holocaust and the strategic bombing of industrial and population centres (such as the devastation of Dresden), as well as military targets (such as the atomic bombings of Hiroshima and Nagasaki), it resulted in an estimated 50 million to 85 million fatalities. These made World War II the deadliest conflict in human history. The start of the war is generally held to be 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, and 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, Poland, Finland, Romania and the Baltic states. The war continued primarily between the European Axis powers and the coalition of the United Kingdom and the British Commonwealth, with campaigns including the North Africa and East Africa campaigns, the aerial Battle of Britain, the Blitz bombing campaign, the Balkan Campaign, the Battle of the Atlantic, and the Normandy landings. Other important participants included the United States, the Soviet Union, China, and Japan. Though Japan had begun expanding into China in 1937, the scale of its activities was significantly increased following the Fall of France in 1940. From late 1940 to early 1941, the United States provided substantial material aid in the form of lend-lease assistance to the Allies. The United States entered the war in December 1941, following the Japanese attack on Pearl Harbor. While the Western Allies fought the Axis powers in Europe and North Africa, the Soviet Union fought a concurrent war against Germanic, Finnic and Japanese invasion in Eastern Europe, particularly in Central Asia. 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, and the Soviet Union's declaration of war on Japan and 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 the Allies to try war criminals, and over a million suspected Axis personnel were detained in Allied occupation zones. The war continued for several more months in the Pacific, until the Japanese surrender on 2 September 1945. The war in Europe had concluded with the defeat of the Axis powers. The Japanese war continued and ended officially on 2 September 1945, with the Japanese surrender. This date is commonly known as V-J Day (Victory over Japan Day), and marks the end of World War II, the deadliest and most widespread war in history.