You're the president. You Know that 3,000 Russian missiles are going to be hitting your territory within the hour. DETERRENCE has failed: for some strange reason (perhaps accident, misperception) the Russians have launched the missiles. 95% of the U.S. population will be dead in 24 hrs. Those who survive will in a sense be the unlucky ones. Nuclear winter (look it up) will be created. The radioactive soup in the atmosphere will kill hundreds of millions North of the equator including much of the populations of Europe, and, yes, Russia.
So the question is, why should the U.S. president do anything?. It seems crazy, but what good would it do to kill 10, 100, or 1,2,3 Billion extra people by retaliating?. So that the U.S. can say "we were tough and strong?" Except that there would be no one around to say it. And most non-marine animal species on earth would be dead within 5 years. answer must be more than 300 words.