The US is a nuclear state, and the only state to have ever used nuclear weapons in warfare. Who's going to defend the world against the threat of American nuclear aggression? Will the world really be safer when only the US has nuclear weapons?
Reducing nuclear proliferation is a noble endeavor, but the United States should not be the one leading the effort towards it as long as it has no intentions of dismantling its own arsenal.
The world would be safer without nuclear weapons, but as long as the US remains an aggressive, nuclear military power that kidnaps, tortures and murders civilians in other countries with impunity, undermines their governments at will, supports terrorist groups and wages war on fabricated evidence, a nuclear-free world will remain impossible.
How about the Indian sub-continent? Nuclear weapons have largely been responsible for keeping Pakistan and India from going at each other. Both know that their enemy has the ability to wipe each other out with nukes.
Nuclear weapons should also be credited with keeping Sino-Soviet tempers from flaring too hot before the end of the Cold War. Some of the border incidents would have easily escalated to full war if Beijing hadn't been intimidated by Soviet nukes.