Constitutionally, I agree he does need permission. However, Congress also passed a law authorizing the president to wage war on any nation that supports terrorism after 9/11. Unfortunately, a rather broad authorization. I don't recall that being repealed. So he may be arguing that.
I agree. After all they pulled our troops out of Iraq claiming the war was over. I think Barry has to go to Congress if he wants renewed fighting in Iraq.
But as you say Congress also passed the POTUS the power to wage war on any nation that supports terrorism. Of course he has to prove that Iraq supports terrorism and since they are busy fighting among themselves right now its a hard one to prove.