Four months ago Europe was still deeply enthralled with covid mania and not interested in pretty much anything else.

NATO forces don't deploy in a foreign country. That's not how it works.

Could Ukraine not have invited them? It's their sovereign right.

But that's irrelevant to the point I was making. You don't stop an invasion by sitting in a neighbouring country, and then staging a counterattack long after the target is overrun. You either stop it at the border, or not at all. Whatever complaints Russia would have raised over Ukraine inviting a NATO deployment, it would have them five times over were NATO to participate in the conflict after it started.

Practically speaking, Russia would have probably backed down. Practically speaking, the reason NATO did not exercise this option is because it doesn't want to fight a war over a non-member.

Tell that the Afghans. NATO could have easily deployed there, or just EU forces. I think people in charge still had hopes to avoid an escalation. And to not get their own troops into the firing line immediately. Deploying troops to Ukraine also limits your options, because it means full, and preemptive, commitment to war. At place and time where the opponent had much more time to properly prepare.

Nato members have deployed in plenty of foreign countries since 1949. Had Nato countries had the strength and will, they would have been able to provide plenty of deterence against the aggression we are seeing today, even if the situation isn't strictly covered by the Nato treaty.