They're created to act as a deterrent; which in itself is a good defence. Some of the other countries who may have designs on us wouldn't dare invade because within the hour they'd have our trident missiles bearing down on their capital. The point is not that they would utterly devastate the city, it is that things would never get to that point. As for proliferation, I haven't thought about it. I'm just happy being in one of the few countries that have them and in a selfish way I'd like it to stay that way :P You say invade look at IRAQ they were invaded because they wanted NUKES to defend themselves...