Currently looks like the U.S. is coming from a position of weakness. So far the only troops the U.S. are sending are to protect NATO assets. The only action the U.S. has acknowledged it will take is sanctions that Russia might feel won't impact them too negatively.
U.S has been in a constant war for nearly 20 years. American citizens overall will not support a new war with any country... But I could envision a scenario where Russia invades Ukraine, U.S. troops are near by in surrounding areas.. Then U.S. stages a false flag operation against American troops, Claiming Russia is solely responsible and that it was intentional..
Americans could be tricked into supporting our involvement if that were to happen.. or at least not resist.
... and then the elephant in the room is how does China react. China would be the domino that would create the global cascade effect if it came to Russia's aid opposing U.S. forces.