>The borders of the USSR have retreated over most of our lifetime, Moscow controlled all of Ukraine as recently as 1990 and gave it up more-or-less voluntarily.

The problem is you think they "own" them.

>If the US can invade the middle east

One nation they invaded was Iraq, and while catastrophic, they've created a democracy that later told US to fuck off: https://en.wikipedia.org/wiki/U.S.%E2%80%93Iraq_Status_of_Fo...

You can very much argue against that, and I'd agree, but it's not comparable to colonial war of aggression to literally enslave and genocide free nation.

>I don't think we need to draw a red line at the Ukraine border against the other nuclear superpower.

They are far from anything super, as we've seen in Kharkiv Oblast last week.