Prevention of the spread of weapons of mass destruction.

the prevention of something increasing or spreading (especially the prevention of an increase in the number of countries possessing nuclear weapons); "they protested that the nonproliferation treaty was just a plot to maintain the hegemony of those who already had nuclear weapons"; "nuclear disarmament and nonproliferation are closely related goals"

The restriction of ability to easily access fissile materials in concentrations sufficient to assemble a nuclear weapon.