This is a maki or break moment for Biden.
Honestly I think the make or break moment happened in 2015 or so, when Obama repudiated the original agreement to defend Ukraine against Russian aggression. Today's events are a logical extension of policy decisions made then.
I think if NATO had mobilized then, and signaled a willingness to stand alongside Ukraine if asked, and made it clear that further Russian aggression would sooner or later butt up against a rubicon of NATO "peacekeepers"... Peace would probably have been kept.
I think Crimea and the eastern enclaves would still have been lost at that time, unless NATO had been really truly ready to get into a shooting war. But mobilizing then, and keeping NATO troops forward deployed in Poland, and actively working on a joint defense plan in case of further aggression, would probably have deterred the further aggression we're seeing today.
Right now I'm kicking myself for not asking this question earlier: Why has all the rhetoric been so one-sided? Why does Putin get to threaten nuclear retaliation and everyone poops their pants and backs down? Sauce for the goose should be sauce for the gander. Why can't NATO threaten nuclear retaliation and Putin backs down?
Even if NATO didn't want to commit ground troops*, there's still a lot else they can offer. Intel, recon, cyberwarfare, air defense, interdiction bombing... Just intercepting Russian aircraft, and bombing their supply depots and rallying points would take a ton of pressure off the Ukrainian army.
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*And it's totally understandable that they wouldn't want to commit ground troops. Joint military ground operations with a foreign nation that shares no equipment or standards or common training and exercises with you is a recipe for disaster.