I think you're mistaken on this. I'm no Constitutional Lawyer, but there is plenty of precedent that authorizes the President of the United States, as the Commander in Chief of the U.S. Military, to order troops to action without congressional approval. The first time this was done was in 1801 by President Thomas Jefferson when he ordered the newly established U.S. Navy to Libya to fight the Barbary Pirates.
But if there are any Constitutional Scholars lurking, they need to fact-check me.
This one, I know you're mistaken on. Congress passed a joint resolution in October of 2002 that authorized military action in Iraq.
That's my fault for wording it wrong. Rather what made it unconstitutional was that it wasn't congressionally declared.
The common argument is the war powers act during Vietnam. The problem is that law in itself was also a constitutional violation -- it embolded executive permission which created an imbalance. The act has been brought up a bunch of times as a dead letter (which I just found out) especially since Clinton bypassed its points when sending troops to Bosnia.
You're right though that congress authorized it, but I should have said didn't declare it. It was unconstitutional.