Russian military doctrine allows battlefield use of nuclear weapons, and the Biden administration has discussed possible responses.

It's a troubling question with no palatable answer: What would President Joe Biden do if Russia used nuclear weapons in the Ukraine war?

A half dozen current and former government officials briefed on the issue, and several outside experts, told NBC News there was no playbook and little agreement about how the U.S. would respond to a norm-shattering act of destruction that could obliterate a Ukrainian city, kill tens of thousands and send a cloud of nuclear fallout drifting over NATO countries in Western Europe.