BAGHDAD — The last U.S. troops have left Iraq, ending the U.S.-led military mission there more than two decades after the 2003 invasion and years after forces returned to fight the Islamic State group. U.S. Central Command said Wednesday that its remaining forces had vacated Erbil Air Base in Iraq’s semi-autonomous Kurdistan Region. About 1,500 U.S. personnel had remained there after American forces withdrew from the rest of the country in January. The anti-ISIS coalition, previously headquartered in Iraq, is moving to Jordan. U.S. forces will continue counterterrorism operations in Syria, according to Central Command. Iraqi officials marked the end of the mission with a ceremony in Baghdad and a military parade. Prime Minister Ali al-Zaidi declared a two-day holiday and called Wednesday “sovereignty day.” In a statement, he described the next phase as one defined by Iraqi sovereignty, stronger institutions and partnerships with other countries. Sabah al-Numan, spokesman for the chief of Iraq’s armed forces, said Iraq was considering bilateral military agreements with the United States. The relationship, he said, would shift from working under a coalition to an equal partnership. The departure follows an earlier U.S. withdrawal in 2011. American forces returned in 2014 at the Iraqi government’s request after ISIS seized large areas of the country and Iraqi army units collapsed. Iraqi forces, Kurdish fighters and the U.S.-led coalition drove ISIS from its territorial strongholds by 2017; the group was defeated in neighboring Syria two years later. ISIS no longer controls major cities, but sleeper cells remain in Iraq and Syria. Iraqi special forces said they killed two suicide bombers near Kirkuk on Sunday before they could detonate their explosives. A civilian was also killed. Al-Numan said Iraq had formed two joint Iraqi-Kurdish brigades for counter-ISIS operations. The end of the U.S. presence has raised concerns among Kurdish leaders, who have relied on American security support and fear attacks by Iran and Iran-backed groups. Al-Numan said Iraq would not leave the Kurdistan Region undefended and was discussing purchases of air-defense systems from South Korea, Turkey and the United States. Iran has long urged Iraq to expel U.S. forces, while Washington has pressed Baghdad to disarm Iran-backed militias operating outside government control. Al-Zaidi agreed during a July meeting with President Donald Trump to disarm the groups by Sept. 30, but has since asked to extend the deadline until next June. Some powerful militias say they will not surrender their weapons while they believe Iraq remains vulnerable. The withdrawal closes a consequential and deeply contested chapter. The 2003 invasion toppled Saddam Hussein, but the ensuing occupation was followed by insurgency and civil war and helped expand Iranian influence in Iraq. At least 200,000 Iraqi civilians were killed, according to the most conservative estimate cited by Iraq Body Count, an independent organization that tracks civilian deaths. More than 4,400 U.S. military personnel were killed, according to the figures cited in the report. Many Iraqis continue to live with the consequences of the invasion, even as Baghdad and other cities have become largely calm. Iraqi officials say they intend to maintain security cooperation with the United States while taking greater responsibility for the country’s defense.