If Putin pushes outside of Ukraine the USA/NATO won't have any choice but stop them, especially if it's in NATO or NATO allied country. I think if goes anywhere outside Ukraine that the USA will start sending troops and Putin won't be able to talk his way out of it or threaten his way out. It will be like Poland in WW2.
Are you so sure it would escalate like that, or is it a bluff?
And if it's a bluff, should Putin be able to bluff his tanks all the way to Berlin?