When a country reduces or eliminates its military weapons, or arms, it's called disarmament. Nuclear disarmament is the reduction of nuclear weapons around the world.
After wars, countries often deal with disarmament, reducing the arms on both sides in a balanced way. Proponents of nuclear disarmament aim for the same thing, often with the ultimate goal being complete elimination of all weapons of mass destruction. Disarmament adds the "not" or "do the opposite of" prefix dis- to armament, from a Latin root meaning "furnish with weapons."