Allies: A term describing alliances of countries in military opposition to another group of nations. Twenty-eight nations made up the Allied and Associated powers in World War I. In World War II, the Allied powers included Great Britain, the United States, and the Soviet Union.

Atomic bomb: An explosive device that releases nuclear energy (energy that comes from an atom's core). All previous explosive devices were powered by rapid burning or decomposition of a chemical compound; they only released energy from the outermost electrons of an atom. Nuclear explosives are energized by splitting an atom, a process called fission.

Cold War: Tense conflicts and failed diplomacy between the United States and the Soviet Union.

Korean War: A war from 1950 to 1953 fought between communist North Korea and non-communist South Korea; China backed North Korea and the United Nations backed South Korea.

Marshall Plan: A post-World...