There is no need to send U.S. troops. Ukraine is a very big country (population over 40,000,000). They have the manpower to do this. What we can help them with is with the military equipment. Of course, we should also be looking at legal avenues for transferring funds from frozen Russian accounts (including their central bank) to the Ukrainians. The reparations for the damage being done to Ukraine's infrastructure and economy (not to mention loss of life and atrocities) by the Russians is going to be very costly.
As I noted there are Russian-speaking populations in many places. Vlad the Dim has used them to stir up conflicts all over the place. It is similar to what Hitler did with the Sudetenland.
But the Russians are not the only ones who can play this game. I've been to Eastern Siberia several times. Many people there do no look Russian. Their first language is often not Russian. Some of the territory in Eastern Siberia once belonged to China or Japan. Ultimately, Russia too will benefit from a general principle that these claims to territory based upon history or the presence of ethnic or linguistic minorities should not be settled by war.