Parliament and Congress Have No Power to Legalize War
Congress has fled town to avoid voting for or against a new war. Many of the big donors to Congressional campaigns would want Yes votes. Many voters would want No votes, if not immediately, then as soon as the panic induced by the beheading videos wears off, which could be within the next month. Better to just avoid displeasing anyone — other than people who notice you running away.
The standard for legal-ish cosmopolitan respectability in the U.S. now has become getting five kings and dictators to say their on your side as you start bombing a new country.