America wouldn't invade they would follow the Cuba missile crisis playbook: destabilize the regime, support opposition factions, and institute economic sanctions. Russia is the only one that has a recent history of invading neighbors.

NATO troops have been in Poland and the Baltics since '04 and there were positive relations between Russia and NATO during the war on terror period. Relations started to deteriorate when Putin decided to brand NATO as anti-Russian in late 2000s. Back then he saw the writing on the wall that a majority of Ukrainian people wanted to align more with the West which threatened his bloc.