You need to read the resolution Congress issued regarding War in Iraq. They ceded their power to use force to the President based on HIS sole discretion. He has their blessing already.
But he doesn't have THIS:
"the President is hereby authorized and directed to employ the entire naval and military forces of the United States and the resources of the Government to carry on war and, to bring the conflict to a successful termination, all of the resources of the country are hereby pledged by the Congress of the United States."