"...and used for justification of the invasion was a UN resolution, as I recall. Or which law do you think Saddam broke that justified the invasion? "

As numerous UN resolutions attest, Saddam did not comply with the cease fire agreement. Sanction were a consequent of Saddam not complying with the cease fire agreement. Had Saddam complied with the agreement the sanctions would have been lifted- and he would have evenually regained his airspace [and be treated as any other State which acted in accordance with international law].
One way to describe it, was that Saddam was put in the doghouse and he chose to remain in the doghouse rather than do anything to end a war which he decided to began.