Then we are morally and legally obligated to defend it based on that treaty we signed back in the 1990s with them in exchange for their nukes” Nope, the Treaty does not say that. The text is above, somewhere, read it.
And the Treaty signed by Ukraine and Russia allowed Russia to have troops.

Anyway, I even don’t know why I am saying this. USA and EU just sit on Treaties and International Borders when it suits them.
I could stop laughing when Obama said “it is illegal”…