An incident in Syria two years ago involving the transport of an Iranian port-a-potty nearly led to a confrontation between American and Iranian forces, underscoring just how quickly even minor events could escalate there.

The episode, told here for the first time, is particularly instructive as the Trump administration signals it might leave behind a small force in both Syria and Iraq to monitor Iranian activities.

Some analysts and US officials believe that the change of mission for those forces could raise the chances of a war between the United States and Iran—and that it may even be illegal under the US Constitution.

President Donald Trump announced in December 2018 that he’s withdrawing all US troops from Syria, but administration sources told Foreign Policy last month that he’s considering keeping a small force at a remote base in southeastern Syria, far from the last remnants of the Islamic State, to counter Iran. And yesterday, Trump said he wants to maintain some troops in Iraq for the same purpose.

“I want to be able to watch Iran,” he told CBS’s Face the Nation. “We’re going to keep watching and we’re going to keep seeing,” he said.

Members of 5th Special Forces Group (A) conducting 50. Cal weapons training during counter-ISIS operations at al-Tanf garrison in southeastern Syria. US Marine Corps

In both countries, the strategy would constitute a core operational change, raising broad questions about the mission. Then-President Barack Obama completed a drawdown of all US forces in Iraq in 2011, bringing an end to the 2003 Iraq War. But the Islamic State’s sweep of broad territories in Iraq and Syria in 2014 prompted the United States to intervene militarily in both countries, alongside a coalition of other militaries, to fight the militant group.

“What is the strategy? What would be the rules of engagement? How would we avoid being sucked into a regional war not of our making?” said Kelly Magsamen, the vice president for national security and international policy at the Center for American Progress. “If I’m a service member in Syria, I would want to know what the heck I was doing there and how my mission fit into a strategy.”

The port-a-potty incident, described here for the first time, took place at a small US outpost called al-Tanf, which sits along a potential Iranian supply route through Iraq to Syria in the southeast part of the country, in May 2017—at a time of heightened tensions across the region.

Just weeks earlier, the United States had launched cruise missiles at the Syrian regime’s Shayrat air base in response to a chemical weapons attack on the town of Khan Shaykhun. The night after the missile strike, on the evening of April 8, al-Tanf itself came under attack from Islamic State fighters. The ensuing battle left three US-backed Syrian fighters dead.

The situation remained tense throughout the next few weeks. On the night of May 9, Russia conducted airstrikes just 14 miles from al-Tanf—close enough that the soldiers could hear the aircraft, according to a US defense official who requested anonymity in discussing internal deliberations. Alarmed, US officials quickly negotiated an agreement with Moscow for advance notice whenever Russian planes strike within a 55-kilometer (34-mile) radius around the garrison to ensure they did not endanger coalition forces.

“The agreement was about airstrikes. But it quickly became our narrative that this is our territory,” the official said about the 55-kilometer exclusion zone.

Days later, a group of pro-regime forces believed to be affiliated with Iran or Lebanese Hezbollah told US commanders, through Russian intermediaries, that they intended to pass through al-Tanf to meet up with a group of Iranian-backed forces in Iraq, who were moving toward the border. The headquarters for the Combined Joint Task Force Operation Inherent Resolve—the name of the joint task force established by the US-led international coalition against the Islamic State to coordinate military efforts against the group—declined to answer the message, a silence Russia apparently took as consent, the official said.

The situation escalated quickly. On May 18, the pro-regime group began setting up a rudimentary outpost just 21 miles from al-Tanf. The coalition headquarters called in an airstrike that killed one person and destroyed two unarmed trucks, a tank, and a bulldozer, the official said (the account is backed up by news reports at the time).

US Marines engage targets during a live-fire demonstration near al-Tanf garrison, Syria, September, 7, 2018. US Marine Corps

US military officials told Russia that the group could remain in place but could not move toward al-Tanf and could not bring in supplies. The US officials told the Russians they would use military force to enforce the directive, the official told FP.

The next day, on May 19, US forces detected a vehicle heading toward the group, carrying a port-a-potty. The coalition headquarters gave the strike order.

The strike never occurred. Air Force officers responsible for operations at the Combined Air Operations Center at Al Udeid Air Base, Qatar—the command-and-control hub of air forces throughout the US Air Forces Central Command region—refused to attack because they did not believe it to be “a lawful order that complied with the rules of engagement,” the official said, describing the idea that a threat was posed to US forces as “ludicrous.”

“They had told us exactly what they were going to do, and we had not told them no,” the official said. “We stray from the Constitution when military commanders choose to use US military force against another state’s force in the absence of a credible, imminent threat.”

Lt. Col. Josh Jacques, a spokesperson for US Central Command, could not confirm or deny the incident. He noted that DOD does not keep records of strikes that do not occur.

“Southern Syria remains a congested and complex battle space,” Jacques said.

A US Marine fires at a target with an M240B machine gun during a live-fire demonstration near al-Tanf garrison, Syria, September 7, 2018. U.S. Marine Corps photo by Cpl. Roderick Jacquote

The incident underscored the tricky legal position US forces find themselves in the region. Under the 2001 Authorization for Use of Military Force—which authorizes the fight against nonstate militant groups such as the Islamic State or al Qaeda—US military forces are not authorized to target state actors such as Iranian, Russian, Syrian, or proxy regime forces in Syria unless they are attacked and are responding in self-defense, noted Melissa Dalton of the Center for Strategic and International Studies.

But Matthew Waxman, a professor at Columbia Law School who served in senior positions at the State Department, the Defense Department, and the National Security Council, said the US presence in Syria has been constitutionally dubious for a long time. Obama’s reliance on the 2001 AUMF to justify the operation was “a big stretch,” Waxman said.

“The Trump administration offered no clear international law justification for air strikes in response to Assad’s chemical weapons use,” he said.

Waxman expects the Trump administration to argue that it is necessary for the United States to keep residual forces in Syria, particularly at al-Tanf, for defense against the Islamic State.

“Does this contradict the President’s statements about ISIS being defeated? Of course it does, but this administration routinely dismisses the President’s own rhetoric as unimportant to its legal position,” he said. “And I wouldn’t hold my breath for a clear and public international law justification.”

US Marines board an MV-22 Osprey on their way to a site near al-Tanf garrison, Syria, September 7, 2018. U.S. Marine Corps photo by Cpl. Carlos Lopez

However, there are signs that, after years of failed attempts to pass a new Authorization for Use of Military Force, Congress will put its foot down on the issue of maintaining a small force at al-Tanf to deter Iran.

“Congress hasn’t authorized an anti-Iran mission in Syria. The truth is the legal basis for the US military presence in Syria, the 2001 AUMF, is pretty shaky and needs to be revisited,” said an aide for Sen. Bernie Sanders, an Independent from Vermont. “Any move by the Trump administration to expand that authorization even more to include operations against Iran will definitely draw a response from Congress.”

Magsamen, of the Center for American Progress, said action by Congress is long overdue.

“We’ve had two years of no real oversight of the wars under this Administration,” she said. “Our mission in Syria has been [counterterrorism]-focused and covered under 2001 AUMF. If that changes to Iran deterrence, that’s a whole new ball game.”

Lara Seligman is a staff writer at Foreign Policy.@laraseligman