I guess we won't really know unless/until an actual NATO member is attacked and Article V is invoked.
we don't need to look that far.
US
already gave Ukraine security/defense assurances as a signatory to the Budapest Memorandum in 1994. A deal was brokered for Ukraine - which at the time had the 3rd largest nuclear arsenal in the entire world - to completely disarm and become a signatory to the NPT - and in exchange Ukraine's sovereignty, borders, and territorial integrity was to remain intact and secure and that the US would guarantee that security. It's not quite Article V of NATO, but the US has weaseled out of living up to their word- but that's just par for the course for us.
Ukraine has effectively been a US colony post the US-backed 2014 coup, and we
still didn't come to their defense in 2014. Putin saw this which is probably we he figured if he takes it a step further now with invasion we still won't send troops into Ukraine.
We're partly responsible for Putin even coming into power. The US literally rigged Yeltsin's election in '96 and then bragged about it afterwards. Yeltsin was Clinton's boy-toy and a client of the US. And who did Yeltsin hand pick as his successor in 1999? Ol' Vlady P.
This is why I say- US needs to stay out of foreign countries internal affairs. It
always has blowback which just winds up biting us in the ass. And why I found it
absolutely hilarious when morons freaked out about the 2016 election over $100,000 worth of facebook ads and twitter trolls lol.