they're not going head on
against the US. Give the russkies some credit, they're not that dumb. They know the minute they launch a nuke, Moscow is the least of their problems. Crimea wasn't even part of the Russian Federation 8 years ago.
That doesn't mean they're not going to try to take over Ukraine though. If they're heavily loading up, is because they know Ukraine is going to get external help.
Ukraine isn't quite like Poland where you can just conquer them by just showing up, but you don't really need a lot from a nation like Russia to take over them.
Russia expectation is that Ukraine is going to get help from "the international community" which is codename for USA/UK/Nato, etc...