One small but important point related to the Non-Proliferation Treaty discussion below
is that, as several readers have pointed out, Israel, India, and Pakistan aren't signatories to the NPT so they can't really be said to be violating it. This is true, but it amounts to much the same thing. The NPT allows any member to withdraw from the treaty with no real penalty -- all it needs to do is to offer three months' notice -- and in the wake of the Bush administration's nuclear deal with India none of the non-member countries are paying any meaningful penalty for their non-membership.
Ultimately, I tend to think the Iran crisis has shown the need to create a much more robust non-proliferation regime than the one the NPT provides for. Getting that done would require a lot of things, among them a renewed determination to deal with the status of the non-NPT Three in a meaningful way.