I know Congress needs to be involved to actually declare war, but there have been a number of times where something was kicked off by presidential authority alone.

If Biden wanted to, could he start a conflict against Russia without congressional approval. If not, what approval would he need? If so, what would be the theoretical limitations to his power and military authority?

I am already assuming people would want some definition of what “conflict” would mean in this hypothetical scenario. So let’s say it means Biden authorized US troops at the Ukrainian border and had them launching shells into Russia.