A host of crucial multilateral interests are baked into the U.S. presence, from keeping the Islamic State down, to protecting vulnerable regional allies, to preventing Iran from taking Iraq's oil revenues.
Iraq suffers greatly from Iran’s interference, but the U.S.-Iraq relationship is demonstrably not a lost cause.
If U.S. troops stay in Iraq, they would greatly reinforce America’s position there and help counter Iran’s malign influence throughout the region. But if they leave, Iraq would be at immediate risk of slipping back into the destructive isolation of the Saddam era, with even less ability to resist Iran’s predatory policies. Most Iraqis rightly dread that thought. The hundreds of thousands of anti-Iranian protestors who have taken to Iraq’s streets in recent months, especially in Shia areas, drive home this point.
Beyond its geostrategic and political value, Iraq is now one of the world’s top oil exporters, with huge reserves for the long term. If the U.S. presence remains intact, the American, Iraqi, and global economy would share in those benefits. If the United States leaves, however, Iran would effectively gain increasing control of vast energy and financial resources, diverting them from Iraqi development in order to evade sanctions and greatly assist its own hegemonic ambitions.
https://www.washingtoninstitute.org/...eed-each-other