When the treaty signed stated you will do x, y, and z and in exchange we will stop pummeling you, then it is exactly a justified reason to put boots on the ground when you don't do x, y, or z. In this case Hussein did not do x, y, and most of z. Iraq lost the war they said they would comply with the terms they signed to in order to have us stop beating them, and if I recall correctly their terms were rather simple, "If you stop beating us we will do what you want." They violated that agreement from the start, so what do you do? You go back to the status quo before the treaty was signed and resume the beating. Diplomacy was tried for 12 years without success, further diplomacy would not have done anything.