I think the line is when your troops shoot their troops.

Providing weapons has been a US-Soviet past time for the whole Cold War.

In this case, I think the US could have even moved in troops without being at war, early in the invasion. It just needs a pretext. By far the best response a week in, when Russia claimed chemical weapons, Nazis, etc., would have been to send in a multilateral force to investigate Russia's claims. It's face-saving for the Russians and ends the war.

Tensions are too high for that now.