World War II (WWII or WW2), in the Soviet Union, the Great Patriotic War, and in Japan, the Second Sino-Japanese War, was a global war involving fighting in most of the world and most countries. Most countries fought in the years 1939–1945 but some started fighting in 1937. Most of the world's countries, including all the great powers, fought as part of two military alliances: the Allies and the Axis Powers. World War II was the largest and deadliest conflict in all of history. It involved more countries, cost more money, involved more people, and killed more people than any other war in history. Between 50 to 85 million people died. The majority were civilians. It included massacres, the deliberate genocide of the Holocaust, strategic bombing, starvation, disease, and the only use of nuclear weapons against civilians in history.
The two sides were the Allies (at first China, France and Britain, joined by the Soviet Union, United States and others) and the Axis (Germany, Italy and Japan). The war in Asia began when Japan invaded China on July 7, 1937. The war began in Europe when Germany invaded Poland on September 1, 1939. France and Britain reacted by declaring war on Germany. By 1941, much of Europe was under German control, including France. Only Britain remained fighting against the Axis in North Africa, the Mediterranean, and the Atlantic. Germany gave up plans to invade Britain after losing an airplane battle. In June 1941, Germany invaded the Soviet Union, starting the largest area of war in history. On December 7, 1941, Japan attacked the United States at Pearl Harbor and invaded British and French colonies in Asia, and the two wars became one.