I agree with jandrese and suzannecharelot, though we may be reaching an inflection point there. Nuclear armament promoted peace as long as any country capable of achieving it had to be wealthy and organized enough to have something to protect (i.e. not want to be destroyed by similarly armed enemies). As we get better and better at physics and engineering, it becomes more plausible for rogue states and non-state actors to develop basic nuclear or radioactive weapons.
The USSR knew if they attacked the US they'd be destroyed, and vice versa. If North Korea attacks Seoul, ditto (or Iran and Jerusalem or Tel Aviv). If Pakistan attached India, India would retaliate but probably lacks the capability for an overwhelmingly certain second strike, and I think the U.S. is unlikely to spark global nuclear conflict over such an event. I doubt Al Qaeda could acquire and use enough material to build a nuclear weapon successfully, but on a fundamental level the assembly is not exceptionally hard. In WWII the US tested the bomb design we used against Nagasaki, but we used the Hiroshima one without bothering to test it, because it was straightforward to design, obvious it would work, and not worth the time and cost of making extra material for a test device. And if you're a suicide bomber, you don't even need a plane to drop it.