In hindsight, of course, the best way to deter the invasion was many years before - 2014 or before. But let’s set that aside and only go back to, say, the beginning or middle of February 2022.
Even without the U.S. or NATO directly intervening in the war, Russia appears well poised to lose; Ukraine has been handing its ass to it ever since Feb-24-2022. Nevertheless, the cost has been enormous; 1/3 of Ukraine’s population has been displaced, over 300,000 have died, who knows how many wounded, a trillion dollars in economic disruption, etc. In other words, in terms of deterrence, a stitch in time could really have saved 9 - indeed, perhaps more like a stitch in time saving 99.
All the way up to the day of the invasion itself, Biden made it very clear that U.S. armed intervention was not on the cards, and that was NATO’s stance likewise. At the same time, in hindsight, it’s abundantly clear that all the political, economic and diplomatic sanctions in the world wouldn’t stop Putin from invading. The West basically threatened Putin with every non-military thing under the Sun and it didn’t work.
Arguably, the threat of armed force (U.S. and/or NATO) was the only thing that would have prevented Russia from invading. One may argue that there is no formal treaty obligation to defend Ukraine as such, but the United States and NATO have intervened in plenty of situations where there was no treaty at stake - Saddam invading Kuwait, the Serbia-Kosovo situation, etc. - and Biden has even gone so far as to say America would defend Taiwan even though China is nuke-armed and there is no formal US treaty to defend Taiwan.
So - in hindsight, would a public, serious US/NATO pledge to intervene directly with military force have prevented a Russian invasion of Ukraine, and if so, was it a blunder in hindsight not to pledge as such?