World War II was a massive global war fought from 1939 to 1945, involving most of the world's nations divided into two opposing alliances: the Allies (including the United States, Britain, and the Soviet Union) and the Axis powers (Germany, Japan, and Italy). It remains the largest and deadliest war in human history.

World War II transformed the modern world. It led to the creation of the United Nations, an organization designed to prevent future global wars. The war's end established the United States and the Soviet Union as superpowers, setting up tensions that would define the next half century. The war's brutality also led to new international laws about war crimes and human rights.