A couple of thoughts (or repeats of thoughts) based on some of the stuff above.
On Biden’s response; think of how it would have looked if Biden had left open the possibility of coming to Ukraine’s aid with military forces. He would now be facing intense pressure from Ukraine and war hawks everywhere. The US response would now be viewed as extremely weak rather than the relatively positive coverage it gets now, Putin would be able to crow that he had called Biden’s bluff.
I was sceptical about Biden’s line as well at the time, but in hindsight, IMO, he was 100% right. You should never bluff if your bluff will be called, and this was always going to be called by Putin. The only way to prevent that would have been to put American boots in Kiev, which would have been a major diplomatic escalation (and I doubt the Ukrainians would even have agreed).
On the subject of how US forces in Ukraine don’t necessarily mean nuclear war, do keep in mind that any war involves two sides. The WW3 scenarios are instructive - NATO (publically at least) imagined the possibility for conventional warfare. Almost all Soviet scenarios involved limited or unlimited nuclear war. There was never a reality where such a war broke out without it turning nuclear.
Again - the Russians had their nuclear briefcase activated after the cold war in a time of peace due to a weather rocket. Actual hot war = nuclear war.
US/NATO in Ukraine would be a significant escalation of the conflict. We step in and turn the tide (because what’s the point of intervening if we don’t?) This will involve hitting Russian military targets in Russia, because Russia will sure as hell shell NATO positions and fly air support missions from within Russia. Now Russia is losing the war.
Where do you think this goes next? Does Putin pull out Russian troops, admit that he is the bad guy, and allow US/NATO attacks on Russian soil remain unanswered? Or does he escalate the conflict further, using more than 5-10% of Russia’s armed forces? At what point does Putin start wondering whether a limited tactical nuclear strike on Kiev could be a better option than losing in Ukraine?
Because you can ask exactly the same question about US troops in Ukraine, to the subject of Nuclear war. Is the US willing to start a full nuclear war, just because Russia dropped a small tactical nuke somewhere in Ukraine? Well… are you?
I’ve read some people (not here, I think) who seem to imply that Zelenski is weak because he is willing to come to the negotiation tables so fast, and seems to indicate he is willing to give Russia what they want at least on paper (Ukrainian neutrality, etc). But that’s kind of the point. Any hope for peace with Putin relies upon being able to give him some result that allows him to save face.
Putin has to win, even if he didn’t actually win. Be prepared for that. We’re going to hear a lot of people scream because they don’t understand that basic psychological game.