World War II was the deadliest and most widespread conflict in human history, involving over thirty countries and lasting from 1939 to 1945. It began with the German invasion of Poland, prompting Britain and France to declare war on Germany. Over the next six years, the war expanded across Europe, Asia, Africa, and the Pacific. Major powers formed two opposing alliances - the Allies and the Axis. The Holocaust, carried out by Nazi Germany, led to the murder of six million Jews and millions of others. In 1941, the United States entered the war after the Japanese attack on Pearl Harbor. By 1945, the Allies had defeated the Axis powers, with Germany surrendering in May and Japan in August following the atomic bombings of Hiroshima and Nagasaki. The war reshaped global politics, led to the formation of the United Nations, and set the stage for the Cold War between the United States and the Soviet Union.