NATO has had a vested strategic interest in border stability in Europe, it's really one of their unwritten rules. When a country starts moving a border using military force, it's pretty serious. It can happen in Africa and they don't get too worried, or Yugoslavia can break up and it's "OK" except for the strife that followed. The core idea is that borders are sacrosanct. Nobody should be allowed to change any of them by force. Crimea worried everyone but there didn't seem to be any option. I think folks figured "OK, it used to be Russia, we'll use harsh naguage and let it go." The idea Russia could basically run over Ukraine and install their own rump government was viewed differrently. When Ukraine resisted, it obviously encouraged support to keep them humming.
This is inherent in Article V, which of course doesn't include Ukraine fortunately.
I think the other interest is to bleed Russia dry and get rid of Putin. "We" are putting in "money" basically and Russia is putting in troops. Ukraine is our mercenary force.
I don't think we'd launch a first strike on Russia and if we did, there is little chance we'd knock out everything, and even ten remaining for counterstrike would be ...