Earlier quoted context omitted.
Starting illegal wars and engaging in extreme corruption, for starters.
The war isn't illegal. The president has that power. I don't like it either, but since the Korean War this is simply a statement of fact.
The problem is: over time the US grew so powerful, that the definition of "war" became blurry. "No, we are not at war, our soldiers are just dropping bombs on Iran for fun and profit".
EDIT: Another problem, of course, is that current member of Congress have no balls to stand up to Trump and reclaim their constitutional powers.