As the Marine Corps general leading U.S. operations in the Middle East in 2013, James Mattis was fired by then-President Barack Obama for his hawkish views on Iran. Today, as defense secretary, Mattis is again in a tight spot due to U.S. policy toward Tehran—but this time, he is on the receiving end of hawkish pressure.

President Donald Trump’s administration has in recent months been ramping up economic pressure and rhetoric against Iran. This week, National Security Advisor John Bolton issued a particularly pointed threat, warning that there will be “hell to pay” if Tehran continues to “cross” the United States and its allies.

“The murderous regime and its supporters will face significant consequences if they do not change their behavior,” said Bolton, speaking at the United Nations General Assembly gathering in New York. “Let my message today be clear: We are watching, and we will come after you.”

In a move that strayed squarely onto Defense Department turf, Bolton declared that the United States will keep a military presence in Syria as long as Iran has forces there. This put him at odds with the Pentagon’s long-standing assertion that the roughly 2,000 U.S. troops are in Syria for one purpose: to ensure the lasting defeat of Islamic State militants.

Mattis now must walk a fine line between the administration’s increasingly hostile rhetoric against Iran, including its regional ambitions, and concerns about U.S. mission creep in Syria.

In reality, Mattis and Bolton likely share similar concerns about Iran, experts say. But the defense secretary has reason to present a more cautious front in Syria. For one thing, lawmakers in Congress are balking at what they see as an illegal expansion of the U.S. mission in Syria, which is legally predicated on a fight against nonstate militant groups, not a country like Iran. At the same time, on the ground, escalation poses a very real risk to U.S. troops from Iranian proxies, forces loyal to Syrian President Bashar al-Assad, and Russian forces.

“Mattis is probably most concerned about the implications for U.S. forces in theater (e.g., blowback from Iranian and Russian proxies) and is being more cautious in his public statements, given Congressional concerns about legal authorities,” Melissa Dalton, an analyst with the Center for Strategic and International Studies, said in an email.

“Mattis may also not want to get out in front of the White House in this political environment,” she added.

In a gaggle with reporters at the Pentagon on Monday, after Bolton’s comments, Mattis asserted that there has been no change in U.S. policy toward Syria.

“We obviously have got to train up local security forces so that ISIS [can’t resurge]. But that is part of the defeat of ISIS,” Mattis said. “I’m not sure that that’s any change. … We’re up against an unconventional enemy, not a conventional force that could come right back in. I think this has been our position for at least a year and a half.”

Still, he insisted that he and Bolton are “on the same sheet of music.”

Mattis’s remarks were not enough to quell congressional backlash. On Wednesday, lawmakers called into question the U.S. military’s role in Syria, grilling Pentagon officials on whether U.S. troops will remain in the country to counter Iran.

The existing authorizations, which the administration has used to keep troops in Syria and Afghanistan to fight militant groups, do not cover operations against Iran, Robert Karem, the assistant secretary of defense for international security affairs, told Congress.

But he stressed that “wherever we are in the world, our military forces have a right to self-defense.”

He also noted that continued U.S. troop presence in Syria has the “residual benefit” of deterring Iran, but he sought to separate U.S. military activities and policy objectives.

“It is clearly a high priority of the United States to counter Iran’s malign activities throughout the region, including in Syria,” Karem said. “I would disaggregate, however, our overall U.S. policy objectives from our military activities.”

But some lawmakers were not buying it.

“That to me sounds like we’re sending our military to Syria to counter Iran, especially because [the U.S. troops’] withdrawal is apparently dependent on actions of Iran, not actions of ISIS or the defeat of ISIS,” said Massachusetts Democratic Rep. Seth Moulton. The mission of deterring Iran in the region would require authorization from Congress, Moulton said.

Later that day, Congress doubled down. A group of senators introduced legislation designed to prevent unconstitutional war with Iran. The legislation would prohibit the United States from expending funds that could lead to war with Iran without express approval from Congress, and it argues that a preemptive attack against Tehran is illegal under the War Powers Act and the Constitution.

“The administration’s approach to Iran is ripped straight out of the same playbook that launched us into the failed invasion of Iraq, and Congress needs to assert its constitutional authority and halt the march to war,” said co-sponsor Sen. Tom Udall, a New Mexico Democrat and a member of the Senate Foreign Relations Committee.

Experts say that in using the 2001 Authorization for Use of Military Force to justify maintaining a presence in Syria after the Islamic State is defeated, the administration would be on shaky legal ground. This argument “borders on tenuous,” Dalton said.

Michael O’Hanlon, an expert with the Brookings Institution, agreed that the administration does not have a strong legal argument for remaining in Syria. But it can still argue that it is “preventing the re-emergence of ISIS.”

“Secretary Mattis may be hewing closely to the ‘defeat ISIS’ rationale because of the legal questions,” Dalton said.

Still, Mattis has made his personal feelings on Tehran clear—including the role it can play in ending the seven-year conflict in Syria.

“Everywhere you go in the Middle East where there’s instability, you will find Iran,” Mattis told reporters. “So in terms of getting to the end state of the Geneva process [for a diplomatic end to the Syrian war], Iran, too, has a role to play, which is to stop fomenting trouble.”