What would happen if Pakistan was part of the Non-Proliferation treaty and began making nuclear warheads?
This would be a rhetorical question, since Pakistan already has nuclear weapons and has had them for some time. And they are not signatories to the Nuclear Non-Proliferation Treaty. But if Pakistan had signed the Treaty and just begun to make nuclear weapons, they would be in violation of the Nuclear Non-Proliferation Treaty. The UN Security Council would theoretically be justified in taking action against Pakistan (such as economic sanctions or military action) to get them to go back to obeying the treaty. See how well that’s working with Iran? Iran has been flouting the Nuclear Non-Proliferation Treaty for years, and Russia’s been right in there helping them to do it. So we have a declared nuclear weapons power signatory to the NNTP (Russia) helping a non-nuclear weapons signatory to the NNTP (Iran) to get nuclear weapons, and the rest of the world’s response has been to threaten Iran and ignore Russia. After all, Russia is also a member of the UN Security Council, and can veto any a