Iraqis celebrated the departure of U.S. forces after more than two decades of war on Wednesday, though many were worried that a security vacuum could increase the power of Iran-backed militias or provide an opening for the return of the Islamic State group.

The withdrawal of the final troops, mainly from an airbase in Erbil in the semi-autonomous Kurdish region, ended the biggest U.S. war of the century, which brought widespread devastation and chaos from which Iraq has yet to fully emerge.

Tehran and its allies hailed the withdrawal as a victory in their campaign to drive U.S. forces from the Middle East.