Ben Hodges (retired American lieutenant general, former commander of the US forces in Europe), was asked that question on Friday I think.
"There is no real advantage on the battlefield, it would be impossible for the US to stand by/not react, and I don't think Putin or his closest advisers are suicidal..The US will react to Russia's use of nuclear weapons because China, North Korea and Iran are watching it. If we don't respond, they will be encouraged. The response will not necessarily be nuclear."
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