Is nuclear retaliation ethical?

Most of you did not live through the peak of the cold war and the threat of mutually assured nuclear destruction. But the Phactor went from ducking and covering and building bomb shelters in basements, to integration race riots, to the Vietnam war. Small wonder most college classes and all grades seemed irrelevant. So what is the USA policy on use of nuclear weapons now? Against who do you retaliate if the attack is from a terrorist organization? What does the USA do if two other nuclear states some of which seem way more prone to rash action start trading big boomers? Is retaliation of the nuclear sort ever ethical? It does not seem so when the consequences are so dire.
In the certainty of some political circles these days it takes guts just to ask the question, and don't expect subtle nuances of reasoning in the answer. Reading this it bothers me anew that any one person should have such power at their finger tips whether you trust our president, this president, or not. And it's not just our concern, USA citizens' concern, because inherent in the presidency is the power to affect virtually everyone. What would the world think if USA voters gave that power to a simple-minded woman from Alaska or that fundamentalist preacher/politician from Arkansas who might well believe in an apocalyptic world view. If a little voice said, "This is God, Mike, I want you to use your power to bring about the apocalypse now." Would he stop and think that was a weird thought, or be true to his faith? Unfortunately the world we live in makes it important to know. Damn, that's depressing.