Germany officially started World War II with the invasion of Poland on Sept. 1, 1939. Japan and China also engaged in war after the July 7, 1937 incident at Marco Polo Bridge near Beijing, China, which is also mentioned as the beginning of World War II.
World War II began during the Great Depression, a period of instability. Intense nationalist feelings and a desire to expand developed in Germany, Italy and Japan. Germany also had harsh terms forced on it at the conclusion of World War I. In 1931, Japan seized Manchuria from China, Italy invaded Abyssinia in 1935, and Adolf Hitler militarized Germany's Rhineland in 1936.
World War II is known as the most destructive war in history, as of 2014. Casualties, including service personnel and civilians, may have totaled more than 50 million, with the Soviet Union reporting 42 million by itself. Germany suffered the loss of 9 million military and civilians, and China lost 4 million. The United States is estimated to have suffered 420,000 losses.
In August 1945, an American B-29 obliterated Hiroshima with an atomic bomb. When the Japanese did not surrender, a second atomic bomb was dropped on Nagasaki. On Aug. 15, 1945, Japan officially surrendered. Six million Jews were killed during the Holocaust, a Nazi push for racial purification.