The groundwork for the Iranian ascendancy was laid by George H. W. Bush, SecState James Baker and U.S. Ambassador to Iraq, April Glaspie in 1990.
When Sadaam Hussein informed Glaspie of his intent to annex Kuwait Glaspie told him the USA wasn’t concerned about Arab vs Arab conflicts.
So Sadaam took that as an indication the field was clear and he invaded Kuwait.
He found out that Glaspie was wrong when the USA responded in 1991 with the Persion Gulf War to remove Iraq from Kuwait.
In so doing Iraq’s military power was virtually destroyed.
Since then the mid-east has been a can of worms for 28 years now, sucking US treasure and killing and wounding thousands of US military men.
The Gulf War and subsequent war and conflict has destroyed what little stability did exist in the area prior to 1990, and as other posters here pointed out. leaving Iran as the supreme power in the area.
In the 90s Cheney gave great reasons for why they didn’t invade Iraq after kicking Saddam out of Kuwait. He said it would turn Iraq into chaos and empower Iran. Don’t know why he and Bush forgot all that. Everything he warned about came true. You can find the old interview with Cheney on YouTube. He makes the same points that Trump makes when he calls the invasion a disastrous decision