National security adviser John Bolton linked the withdrawal of U.S. troops from Syria to the departure of Iranian forces operating there. (NICHOLAS KAMM/AFP/Getty Images)

White House national security adviser John Bolton said Monday that the United States wouldn’t be leaving Syria so long as Iranian forces continued to operate there, suggesting the Trump administration had embraced an expanded mission in the embattled country beyond the defeat of the Islamic State.

Bolton made the comments to reporters in New York ahead of President Trump’s planned speech Tuesday to the United Nations General Assembly, directly linking any future withdrawal of American troops from Syria to the departure of Iranian forces from the nation. Iran has joined Russia and the Lebanese militant group Hezbollah to back Syrian leader Bashar al-Assad in a war in its eighth year. 

“We’re not going to leave as long as Iranian troops are outside Iranian borders and that includes Iranian proxies and militias,” Bolton said, according to the Associated Press.

For years, the Pentagon has said the U.S. military mission in Syria would conclude once U.S.-backed forces defeated the Islamic State and secured the territory the group once occupied. Countering Iranian forces hasn’t been part of the U.S. military mission to date. So far, the military hasn’t received orders from the White House to alter its operation in the country to counter Iran.

Defense Secretary Jim Mattis, speaking to reporters at the Pentagon hours after Bolton’s remarks, said the U.S. military mission in Syria remained focused solely on the defeat of the Islamic State. 

“Right now our troops inside Syria are there for one purpose, and that’s under the U.N. authorization about defeating ISIS,” Mattis said. “Our troops are there for that one purpose.”

Mattis said both Iran and Russia had contributed to instability in Syria, and added that U.S. forces wouldn’t simply be leaving abruptly and exposing people recently freed from Islamic State rule to a resurgence in violence. 

The comments from the two top national security officials raised questions about whether the Trump administration had adopted a new policy on Syria. Theoretically, the U.S. military could officially stay in the country on a mission to defeat the Islamic State and prevent its reemergence — but still tie its departure to an agreement by Iran to remove its forces from the nation.

The Pentagon’s years-long campaign to empower and assist local Kurdish and Arab militias in their fight against the Islamic State in Syria recently reached its final showdown in southeast Syria’s Euphrates River valley, where the Sunni extremist group’s remaining holdouts are facing pressure from U.S.-backed fighters and airstrikes.

This year, President Trump demanded an immediate withdrawal of U.S. troops from Syria. He later instructed his national security team to prepare for a withdrawal in the near future once U.S.-backed forces succeeded in finishing off the Islamic State.

But in recent months, that policy has appeared to be changing to support a longer-term U.S. military presence in the country.

In addition to seeking a defeat of the Islamic State, the president agreed to an indefinite effort in the country with the added goal of ensuring the exit of all Iranian military and proxy forces from the war-torn nation, according to James Jeffrey, Secretary of State Mike Pompeo’s representative for Syria engagement. Jeffrey said this month that American forces would remain in the country to ensure an Iranian departure, a sentiment Bolton seemed to echo Monday.

“That means we are not in a hurry,” he said, adding that “I am confident the president is on board with this.”

The possible shift in the Trump administration’s approach came as Bolton, a long-standing Iran hawk, took over from H.R. McMaster as the president’s top national security adviser. Bolton advocated for the administration’s withdrawal from the Iran nuclear accord and criticized the Obama administration for crafting a strategy against the Islamic State that risked bolstering the position of Tehran.

“The result is that, today, as the ISIS caliphate disintegrates, Iran has established an arc of control from Iran through Iraq to Assad’s regime in Syria to Hezbollah in Lebanon,” Bolton wrote in October 2017 before joining the White House. “If this disposition of forces persists, Iran will have an invaluable geostrategic position for possible future use against Israel, Jordan or the Arabian Peninsula’s oil-producing monarchies.”

In a speech last year before he joined the White House, Bolton said the declared policy of the United States should be the overthrow of the regime in Tehran. “The behavior and the objectives of the regime are not going to change,” he said. “And therefore the only solution is to change the regime itself.” But at a briefing Monday, he said regime change in Iran is not the Trump administration’s aim. Rather, he said, the administration is seeking “massive changes” in Iran’s behavior as a result of maximum pressure from the United States.

With Bolton coordinating national security policy, the Trump administration could use the approximately 2,000 American troops in Syria as leverage to blunt Iranian influence.

The legal basis the U.S. military relies on to operate in Syria is a 17-year-old law from Congress that authorized the American armed forces to go after the perpetrators of the Sept. 11, 2001, attacks, namely al-Qaeda and any associated organizations. The military considers the Islamic State, which grew out of al-Qaeda in Iraq, an associated organization. The authorization says nothing about Iran. 

When asked at the Pentagon this month about the possibility of extended military presence in Syria, Mattis said the administration remained aligned on its policy but didn’t say the military had expanded its mission to include ejecting Iranian troops and militias from the country. 

“We’re working in Syria,” Mattis said. “As you know, right now the authority is to go after ISIS, and that’s what we're continuing to do.”

Mattis said it was up to the president to decide whether American forces should stay in Syria longer-term as a bulwark against Iran, saying he was prepared to give his views on the matter and suggesting a decision on a longer-term presence hadn’t been made. 

“I'm ready for a conversation,” Mattis said. “But that would be a decision by the president of the United States.”