Can the U.S. deter Putin from using his tactical nukes in Ukraine?

The scenario U.S. officials and outside experts worry most about is this:

If Putin faces a humiliating defeat in Ukraine, he might order the use of tactical nuclear weapons against military units or cities to try to shock the Ukrainians into surrendering…

Stanford nuclear scholar Scott Sagan has suggested another step — private warnings to Russian military leaders that they would be held responsible for war crimes if they used tactical nuclear weapons against civilian targets.

“The United States has a long history of hunting down war criminals,” he told me. “Russian generals may be reluctant to cross the nuclear threshold … and the United States should reinforce that reluctance by adding very personal reasons for restraint.”