Yes he does have to justify war to congress but ultimately to the American people.
Otherwise why make speeches? Too bad the entire process isn't done in secret. Couldn't very well accuse him of lying then.

The out of retirement senior member of the "liberal media" Cronkite has a good column today about the U.S. eating crow with the U.N. thanks to this administrations miscalculations. "This might be about as embarassing a position as this nation has ever suffered in international affairs."