World War II was the largest and deadliest war in history, involving more than 30 countries with around 50-85 million deaths, the majority being civilian casualties. Sparked by the 1939 Nazi invasion of Poland, the war dragged on for six years until the Allies defeated the Axis powers of Nazi Germany, Japan, and Italy in 1945. Battles were fought on the continents of Africa, Asia, and Europe. The United States joined the Allies in 1941 after Japan bombed Pearl Harbor. The rest of the Allies primarily consisted of the United Kingdom, France, China, Poland, and the Soviet Union. After the war ended, the United Nations was established to maintain international peace, security, and cooperation.