Except they can also stop it by bombing western Ukraine which is supplying weapons to eastern Ukraine. They have more options than we do.
The question comes down to this: is NATO defensive, or offensive. In the mid 90's when the Serbs were kicking around in Bosnia, NATO was defensive. IFOR in 1995 was defensive. In 1999 NATO became offensive thanks to Mad Maddy's childhood grudges and Clinton's wag the dog. That was the end of NATO as a plausible defense for Eastern Europe.
“Except they can also stop it by bombing western Ukraine which is supplying weapons to eastern Ukraine. They have more options than we do.”
That doesn’t actually stop anything though. It might slow it down, but they’ll still be dying, probably for a long time to come.