Just Cause

you need just cause as an important condition for waging war. we speculated that Iraq had weapons of mass destruction used this as just cause, and then we went an invaded Iraq. when we found out that their was no weapons of mass destruction. we say that the Iraqi people need us. its obvious we are not going to pull out of Iraq while the president is in office. what i don't understand is how this war falls under Jus Ad Bellen their really was no just cause, no possessing right intention. and who is the proper authority ? "Congress" i guess what I'm trying to say, what makes this war just.