Actually, they seized part of Ukriane, bloodlessly, in 2014. Then they have been engaged in hostile activities in Ukraine in the Donbass region for several years. So, an invasion wasn't shocking.
In the larger sense, though, why does it help American interests to have a proxy war against Russia? In the last two wars in Europe, they were actually our ally. And we currently have no specific reason to be at war with them in any case. They have become a favorite villain of the left, but that's more propaganda than actuality.
Why is Russia our enemy? And why are we spending billions of dollars, (that we don't have, and will be added to our debt) to fight Ukraine's enemy? Why are Ukraine interests our interests, all of a sudden?
Russia's attack on Ukraine violates the international norms. It violates what the UN was developed to try to stop. An attack on one sovereign country is an attack on all. It sets a horrible precedent if left unchallenged.
We weren't looking to have a "proxy war with Russia". Russia started this and we are just helping Ukraine. Just like the rest of the world is helping because they recognize what Russia is doing is wrong and evil.
There are now 6 million refugees from Ukraine. And who knows how many deaths. How big does a war have to be, before you are willing to stand up to an agressor?