U.S. military forces must remain in Syria to prevent the resurgence of the Islamic State or another terrorist group, Secretary of State Rex Tillerson said Wednesday.

“It is crucial to our national defense to maintain a military and diplomatic presence in Syria, to help bring an end to that conflict and assist the Syrian people as they chart a course to achieve a new political future,” Tillerson said during an address at the Hoover Institution in Stanford, Calif.

U.S. forces will remain “focused on ensuring ISIS cannot reemerge,” Tillerson said, but he emphasized that the threat of terrorism flows out of a medley of other dynamics in country. In particular, he cited Syrian President Bashar Assad’s inability to maintain power absent foreign support, and the threat posed by Iranian military expansion throughout Syria. U.S. lawmakers have long expected Iran and Russia to attempt to force an American withdrawal from Syria following the defeat of ISIS as a landholding terrorist group. Tillerson maintained that won’t happen.

“In short, Syria remains a source of severe strategic threats and a major challenge for our diplomacy, but the United States will continue to remain engaged as a means to protect our own national security interests,” Tillerson said.

Iran’s involvement in Syria is a major concern. “As part of it's strategy to create a northern arch stretching from Iran to Lebanon and the Mediterranean, Iran has dramatically strengthened its presence in Syria by deploying Iranian Revolutionary Guard troops, supporting Lebanese Hezbollah, and importing proxy forces from Iraq, Afghanistan, Pakistan, and elsewhere,” Tillerson said. “Through it's position in Syria, Iran is positioned to continue attacking U.S. interests, our allies, and [destabilizing] the region.”

To Americans “skeptical” of an ongoing presence in Syria, Tillerson recalled that the Islamic State formed out of the remnants of al Qaeda in Iraq after the full withdrawal of U.S. troops in 2011. “We cannot allow history to repeat itself in Syria,” he said.

But he also made clear that U.S. policymakers define that counter-ISIS mission in terms broader than terrorist-hunting. He called for a United Nations-led peace process that would result in the departure of Assad, whose effort to maintain power through a civil war allowed for the “ungoverned spaces” that ISIS exploited. Such an outcome would also blunt the Iranian threat.

“Reducing and expelling malicious Iranian influence from Syria depends on a democratic Syria,” Tillerson said. “For many years, Syria under Bashar al Assad has been a client state of Iran. A Syrian central government that is not under the control of Assad will have new legitimacy to assert its authority over the country.”