... is not a re-energised Russia, nor a nuclear Iran, and certainly not a superpowered China. All these things could be lived with and accommodated.
You can't, BTW, go around blowing up every state which looks to acquire nuclear weapons - that's what Teller and von Neumann wanted to do back in the late forties, when the USSR went nuclear. And no, a nuclear Iran will not blow up Israel - they're not prepared to 'martyr' their entire country, which would be the sorry consequence. Remember MAD?.
No, none of the above is truly scary. But how about that thin secular-Islamic political class in Pakistan fracturing and dissolving in a vortex of fundamentalism, and the Taliban getting hold of sixty nukes?
Now that's scary.