Bernie should be aware of the war powers act. It's one of the worse pieces of legislation ever, but it makes the whole declare war thing largely meaningless.

The act gives a president the ability to perform military actions provided Congress is notified within 48 hours of the action happening. Then the president gets a free 60 days to do whatever without additional approval. Then there's a further 30 days where forces should be withdrawing if there is no further congressional approval. However, that timeline doesn't really matter, as the Supreme Court ruled under Clinton that of troops are gone by the time the case gets to them then it doesn't really matter that the law was violated.