I suspect we'd be threatening them, but we'd be doing it without the support of International Law. Hasn't stopped us in the past, but the global reaction would certainly be different. We didn't bomb Pakistan, after all.In terms of Iran, I don't actually think it makes a difference. If they weren't part of the treaty, do you think Israel and the US wouldn't be threatening them?
Military action sanctioned by the UN would be a difficult sell, whereas we can currently use the NPT to help sell such action.