I believe it was purely for reasons of expansion. I don’t know exactly what our response should have been, but we need to at least acknowledge that Ukraine is defending their sovereignty and Putin/Russia are the aggressors. We should support them at least minimally.
The US did not start this war. Putin/Russia started this war by an unprovoked attacks on Ukraine. It is a Ukrainian decision for conscription, the US and Europe are just helping arm and train them.
Any peace deal requires a return of Ukrainian lands to Ukraine because the last time Ukraine accepted the loss of Crimea, Putin reneged on his word and attacked again.