How has the world curbed countries on nuclear weapons

States who have nuclear weapons are mostly signatory states of the NPT which is an agreement on them. The idea behind it is that countries should remove their weapons and dismantle them or at least reduce the amounts of rockets. If a country had nuclear weapons and was considered dangerous, like it was believed for Iraq, it would be invaded to stop the regime. Countries also fear mutually assured destruction, so nobody actually attacks anyone with them because they know what would happen.