I didn't know that waiting around and asking for permission to fight a War on Terror from the U.N. and worrying about the opinions from France, Germany, and Russia was in the U.S. Constitution.
It must be right next to right to engage in sodomy or restrict political groups via Campaign Finance Reform.
The President doesn't need permission from the UN. Congress can only declare war. The President can use the War Powers Act, but that would not happen in the scenario you have suggested.