President Trump’s surprise decision to pull U.S. troops from northern Syria will hurt U.S. intelligence-gathering, erode U.S. credibility, help Iran arms its proxies in Lebanon, and help ISIS regrow, says a retired three-star who helped build up Syrian rebel forces.

Michael Nagata, who retired in August, has served as the Joint Staff’s deputy director of special ops and counterterrorism; commanded Special Operations Command Central, or SOCCENT, and played a key role in the Pentagon’s early efforts to train and equip the Syrian Democratic Forces to fight ISIS. He also worked as the head of the Directorate of Strategic Operational Planning at the National Counterterrorism Center.

The withdrawal, in Nagata’s view, would help Iran cement control of the Shia Crescent, a region that stretches from Afghanistan to Lebanon. He said the U.S. presence in northern Syria had disrupted Iranian activity across a vital portion of that territory. “It wasn’t our strategic goal” to do that, he said. “But it did have a corollary strategic benefit from our point of view.”

Now, he said, “My speculation is that it's absolutely rational for the Iranians to think, ‘The Americans are leaving. What opportunity does this give me?’”

The departure would also help Russia and others “who believe that they profit from eroded American credibility or influence,” he said.

The announcement opens questions about the some 11,000 ISIS detainees being held by SDF forces in Syria. President Trump tweeted that Turkey “must watch over the captured ISIS fighters and families.”

Says Nagata: “There is not enough recognition that this could go horribly wrong in so many ways.”

The United States and other partners could lose a hugely important opportunity to interview those detainees about ISIS’s tactics, resources, and plans. U.S. forces were working to interview detainees and search their electronic equipment for intelligence, but the limited American presence and difficult operating conditions makes that a difficult and time consuming process.

“We’ve been trying. Certainly, there have been good people trying mightily to get into this part of Syria and then with the cooperation of the SDF do what I’m talking about. But we’re talking about thousands of people. We’re talking about very limited capacity to put more Americans or anybody else on the ground,” he said.

It would be almost “impossible to calculate” the intelligence loss that would occur if those detainees were either killed or released, he said.

This loss will be a boon for ISIS, which despite Trump’s repeated claims, is regaining people and power. A Pentagon report from August says that ISIS currently has resurgent cells operating in Syria.

Some estimates suggest that there are as many as 18,000 ISIS fighters still active in Iraq and Syria. “Numerically, it’s still much larger than al-Qaeda in Iraq ever was in its heyday …. And Al Qaeda in Iraq darn near brought down the government in Iraq,” Nagata said.

ISIS has become harder to fight, “Because it’s reverted to an insurgency-style operation,” he said. "It was a hell of a lot easier when they were fighting in large formations out in the open where we could see them and strike them with impunity. Now they’ve mingled with the population.”

It’s exactly the sort of challenge that would be easier to overcome with valuable intelligence from detainees, intelligence that is now in jeopardy.

The move is also likely to undermine an already shaky Baghdad government, to ISIS’ benefit. The past week’s clashes between the government and protestors have left more than a hundred dead and more than 6,000 injured.

The chaos in Iraq “compounds the opportunity for ISIS to be a very effective insurgent actor in this part of the region,” said Nagata. “If you’re a different franchise Suni Arab in western Iraq, such as Al Anbar, you have to be looking at what’s happening in Baghdad and thinking, ‘ I knew it. Despite all of the appeals from the Iraqi government in the last four years to join with them and fight ISIS, and I did it, the Iraqi government is still going to betray me.’ So the next time an ISIS recruiter shows up in your village, you’re probably not going to throw him out. You might not necessarily join him, but you might be thinking ‘Maybe ISIS wasn’t so bad after all.”

Moreover, said Nagata, “If I were a surviving ISIS leader and watching the American announcement that they were leaving northern Syria, and watching the political chaos and violence in Baghdad, I would be turning to my colleagues and saying ‘We’re not done yet. I see opportunities to recover and rebound.’”