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.
You say that but that isn’t how it would happen.
There would be months or years of prep work, spreading propaganda that Canada was the source of our woes, that they were wronging us. By the time we invaded there’d be just enough “legitimate discourse” about the invasion that the Presidents supporters could claim any effort to stop him was political.
There was a time not long ago where people said you couldn’t do lots of things or you’d get thrown out - then Trump did many of them, even got impeached (twice!) and stayed in office. In practice, these limits are at best inconvenient for a dedicated lunatic.
No analogy is going to be perfect.