The United Nations was established in 1945 to replace the League of Nations, which had failed to prevent World War II.

The Soviet Union suffered an estimated 27 million casualties during World War II, more than any other country.

Little Boy was the name of the atomic bomb dropped on Hiroshima, Japan, on August 6, 1945.

The United States was the first country to develop an atomic bomb, which was tested in New Mexico in July 1945.

Auschwitz-Birkenau was the largest Nazi concentration camp and extermination center, where an estimated 1.1 million Jews were killed.

Poland was the first country to declare war on Germany in World War II, on September 1, 1939.

Operation Overlord was the name of the Allied invasion of Normandy in June 1944, which led to the liberation of France.

Japan was the last country to surrender in World War II, on September 2, 1945.

The Treaty of San Francisco was the peace treaty that formally ended World War II between the Allied Powers and Japan.

The United States experienced the greatest economic growth in the aftermath of World War II, due to its strong industrial base and the Marshall Plan.

The Truman Doctrine was the Cold War policy of containment, which aimed to prevent the spread of communism.

The Soviet Union was the first country to launch a satellite into space, Sputnik 1, in 1957.

Neil Armstrong was the first human to walk on the moon, on July 20, 1969.

The United States was the first country to develop a hydrogen bomb, which was tested in the Pacific Ocean in 1952.

The Cuban Missile Crisis was a 13-day confrontation between the United States and the Soviet Union in October 1962, during which the two superpowers came close to nuclear war.