World War II (WWII or WW2), also called the Second World War and, 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 Germanyinvaded 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.
The Japanese victories were stopped in 1942, and in that same year the Soviets won the huge Battle of Stalingrad. After that, the Allies started to fight back from all sides. The Axis were forced back in the Soviet Union, lost North Africa, and, starting in 1943, were forced to defend Italy. In 1944, the Allies invaded France, and came into Germany from the west, while the Soviets came in from the east. Germany surrendered on 8 May 1945. Japan formally surrendered on September 2, 1945. The war ended with the Allied victory.
After the war, the United Nations was set up to develop support between countries and to prevent future wars. The Cold War among the major winners soon started, but they did not fight each other in an actual war. Decolonization of Asia and Africa, where those countries controlled by European countries were given their independence, happened as well. This was because European power was weakened from the war. Economic recovery and the political integration (the process of uniting countries) were among other results of the war.