A nuclear war or atomic war is a war which involves the use of nuclear weapons, widespread or restricted. This has only happened once in human history, when the United States used nuclear bombs on the Japanese cities of Hiroshima and Nagasaki at the end of World War II. A nuclear war where both sides are armed with nuclear weapons would likely devastate Earth.
There are, hypothetically, two types of nuclear war:
Limited war refers to the use of tactical nuclear weapons against enemy strongpoints.
Full-scale nuclear war refers to the widespread use of strategic nuclear weapons against enemy cities.
It is often the plot of movies and games, such as the Fallout series and the movie Dr. Strangelove.