failure to defend Ukraine will lead defending the US, which will lead to nuclear warfare

What? That makes zero sense. You do realize that when the US, a nuclear power, enters troops into the Ukrainian conflict backing Ukraine, Russia, another nuclear power, will directly declare war on the us and that war will very quickly escalate into a nuclear exchange.

I think you're trying to say in some deranged way that if the us does nothing Russia will somehow decide to keep going (even though it's proven they are having difficulty in Ukraine) and pick up more countries until they get to the us and decide to nuke it?

You seem to think this is exactly like WW2 and Hitler but only superficially. Like you heard the WW2 talking points and distilled them into perfectly fitting here. Which yes, one country invading another is a similarity. However it's a similarity with almost literally every conflict in the world.