A nuclear weapon is an explosive device the destructive potential of which derives from the energy that is released by the splitting or combining of atomic nuclei. The use of nuclear weapons became a threat to the world with the bombing of Hiroshima and Nagasaki to end World War II. The United States federal government worked to aid nuclear preparations throughout the nation. Throughout approximately forty years of American history, tensions between the United States and Russia led to civil defense programs that prepared for a nuclear attack. The government set federal restrictions in order to establish guidelines for states to follow when preparing for a nuclear crisis.