Putin doesn’t want to use nuclear weapons. If he did, he risks the destruction of his country. Putin, like the Soviet Union in history, uses nuclear weapons as bluster to threaten countries whose leaders and people believe the likelihood of Russia using nukes is higher than it is. Trump is scared. He’s happy to use or threaten military force against countries without nukes, but Russia invades its neighbor in a war of conquest while committing many atrocities, and Trump is obsessed with peace, even, at times, weakening support to Ukraine to appease Putin. Letting Russia gain things with nuclear threats increases the incentive for other countries to get nukes, and one of those countries may be more willing to actually use them.