I’m just thinking aloud since I’m not familiar enough with the law of war or the War Powers Act/other shenanigans to confidently say what Biden issuing orders to intervene would do.

Clearly, though, it would be a declaration of war at the behest of a foreign nation, so I would hope it might be the final straw. It lay also explain why he dragged his feet on issuing sanctions on Putin directly/has only issued sanctions thus far.