The Great War was a global thermonuclear war caused by the Sino-American war. It resulted in the destruction of all participating nations and abroad, global climate disturbance and billions of victims as a result of nuclear explosions, radiation exposure and the consequent collapse of social and governmental structures, all within a span of just two hours. There is no evidence that any entire country has not been affected by the war, but there are some areas that avoided nuclear consequences. In the Point Lookout DLC for Fallout 3, Point Lookout State Park is described as a place where bombs didn't fall, but the world left them behind.
Today, the United States faces the threat of nuclear disaster not only from other countries, such as North Korea, Iran, or other dishonest nations, but also from terrorist groups, who could easily access the materials and information needed to build a nuclear weapon.