Everything has a reason:
Iraq '91 was a defense of an ally, which was invaded by an indebted neighbour, to easier repay his debt, incurred in Iraq-Iran war. Even before Iraq's invasion on Kuwait, it has become evident to some (this you know if you read some books about that period-particularly books about Saddam) that the bulwark Saudi's had built against Shia Iran is becoming too much of a menace.
So, Iraq, if you believe some, was forced virtue of denied further loans, into a default, or into procuring funds to repay debt in whatever way it could. Saddam chose second option, he saw Kowait as apropriate, even conjured up some myth about 19th Iraqi region.
Desert Storm came, but Bush Sr. failed to advance and hammer the Iraqi defeat and transform it into demise of the Baath and Saddam. At that time, US was still wary of being seen as the exploiter of the only superpower status.
So, sanctions came, with them hopes Iraqis would rebel, but Saddam got progressively harder and brutal. They kind of muddled along, until Bush Jr. decided for whatever reason (i think it was a combination of spoils of war and Saudi urging spiced with a bit of vendetta for Saddam trying to assasinate his father) to go in and topple the Baath once and for all.
If it was indeed in Saudi interest (at the same time rooting for Iran sanctions), they sorely miscalculated the follow on effects, (which wouldn't be the first time, recall what i wrote earlier about Saddam and bulwark against Iran), they most likely thought Sunni's would come on top again, but then Americans screwed up by disbanding post invasion, Iraqi police and army, and thus destroying the entire structure of the country, creating a vacuum in which various oufits sprung into existence.. Some of the same police and army personnel would later join remnants of Baath fighting to reestablish Iraq under their rule and some of these same Baathists are said to be behind ISIS.