During the cold war two superpowers amassed enough nuclear munitions to render the entire planet unsuitable for human life. The usual excuse for this poor behaviour is this concept of a nuclear deterrent. If you have an enemy who has nuclear weapons, the only way to guarantee that he will not destroy you is to also have nuclear weapons. Mutually assured destruction. A mexican standoff. Through luck or design, this policy worked excellently when the world had two nuclear superpowers focused entirely on eachother (unless you lived in Korea or Vietnam or Afghanistan or any of the other third states we dragged in for limited conventional warfare).
But today we have a different situation. It sometimes looks as though we are gearing up for a big war: U.S.A. vs. the rest of the world. And we have nuclear weapons. We have demonstrated a concrete willingness to unilaterally invade nations for the slightest perceived slight. We are even publicly PLANNING A PREEMPTIVE NUCLEAR STRIKE AGAINST A SOVEREIGN FOREIGN NATION.
The trouble is that the basic rules of the game haven't changed. When you are faced with an enemy who has a nuclear capability, the only way to guarantee that he will not attack you is by obtaining a nuclear deterrent of your own. If we believe we can invade Iran with minor losses, we might do it. But if we believe that invading Iran creates a concrete possibility that New York City will fall to a nuclear warhead, we will think twice.
So it's no surprise that now every potential future enemy of the United States is actively pursuing a nuclear weapons program. It is inevitable that as long as the U.S.A. has nuclear weapons and tries to control foreign governments, everyone will be rushing towards a multilateral mexican standoff.
Now, in the name of nuclear non-proliferation, we've declared that any nation seeking nuclear power is worthy of destruction. We've created the environment in which it is necessary for other nations to have nuclear weapons and simultaneously created an environment in which it is necessary for nations that want nuclear weapons to be our enemies.
Or, to put it another way, we've laid the groundwork for World War III.