Kyiv, Feb.15.– Since the 1950s, the US has created scenarios and war games centered around nuclear war. Nearly all of them suggest that there is no turning back once one begins. The United States President is authorized to employ military force against any 
country he considers appropriate under the Military Use Authorization Directive (MUAD). In this scenario, Putin's assault on Ukraine would be covered by the order, and the US would be permitted to use military force.
It's important to note that President Obama modified MUAD in 2012, including a provision that permits preemptive strikes. There are never any winners, even if they choose a flexible response approach. The MAD (mutually assured destruction) scenario makes it clear that utilizing nuclear weapons comes at a high cost.
However, NATO, not the US, will respond if the insane dictator dares to start a nuclear war. All of Russia's missile silos and nuclear warhead storage sites will be destroyed by missiles from NATO. They might turn to destroy Russian air defense systems, tactical operations centers, and armor on captured Ukrainian territory, in addition to sinking the remaining Russian Black Sea fleet. This expected response has already been conveyed to Russia by the U.S. in an ambiguous manner, which is one of the main reasons Putin hasn't yet authorized nuclear weapons! Putin probably isn't so crazy and knows full well that any nuclear assault on the west would trigger a nuclear backlash and lead to the obliteration of Russia as a nation.
Nevertheless, a big Russian offensive in the spring is to be anticipated, and once the snow melts, Ukraine will need fighter and bomber aircraft with qualified pilots to halt the tide. Zelensky has been desperately trying to get this, but the response is ambiguous and coming in far too slowly.