I have never doubted that George W. Bush sincerely believed that Saddam possessed WMD. No one, including W. could be stupid enough to justify a war on WMD grounds, knowing he would be skewered if relatively little was found. I suspect he thought the US could control Iraq, and thereby protect Israel and the Persian Gulf oil states and contain Iran. Not a bad objective of itself, but a gigantic undertaking.
Bush completely failed by not thinking through, before starting, how to manage the aftermath of the military phase. On his watch, idiots made bad fundamental decisions that set the stage for a prolonged insurgency and continuing state of multi-faction civil war (notably by disbanding the Iraqi army instead of co-opting it, and not imposing a federal system). If Bush wanted to destabilize the ME, his post-war management did a heck uv a job.
He needed to follow up the Iraq war by destabilizing the Iran regime, and helping with its overthrow. It was ripe for that, so long as Americans didn’t invade. Instead, Bush focused on Iraq, and didn’t retaliate when Iran killed American soldiers on a daily basis. Ridiculous. If that was the plan, he should have just killed Saddam, replaced him with a Baath stooge of our own, and left town to leave the Iraqis and Iranians to kill each other.