The interest is obvious: stopping expansion and further aggression from Russia. If Russia is not stopped here, he will continue invading countries-- even NATO, which they are already threatening to do, though they talk about "rebellions" that might suddenly occur "hint hint".
I'm surprised you even need to wonder what our interest is.
Russia is not the Soviet Union. Containment was against communism. Containment of or attempted containment of normal great power behavior in a corner of eastern Europe has no , to me, compelling interest component for the US. If it is a problem for anyone it would be European countries. The return of the domino theory does not stir me. At most it may be of some interest to the US to provide enough material support through intermediaries to keep the Russians and Ukrainians fighting among themselves so that potential Russian-Polish conflict can be discouraged.