Of all the problems President Donald Trump’s withdrawal from Syria might cause, the one with the clearest consequences is a resurgence of the Islamic State and the terror attacks it inspires. 

Increased Russian or Iranian influence in the region, for instance, is incremental and might be in the eye of the beholder. A terror attack on the other hand is real, immediate – and often deadly.

The Kurdish-led militia allied with the United States has been, among other things, acting as a jailer. It was guarding about 20 facilities holding roughly 10,000 suspected Islamic State members and camps for another 70,000 family members. In the chaos of Turkey’s invasion of northeast Syria, the Kurds say they’ve had to choose between fighting and guarding those camps. Some prisoners escaped, they say, and some family members were able to leave the camps.

Experts say the Islamic State already had been quietly reconstituting itself in Iraq and Syria, even while under intense military pressure, and that Turkey’s invasion is almost certain to give it a big boost.

During the debate of Democratic presidential candidates last week, former Vice President Joe Biden said Trump’s decision would lead to new terror attacks in the United States. The Islamic State is “going to come here,” he said. Trump says a resurgence of the Islamic State is not his problem – it’s Europe’s. Aside from this selfish disregard for countries that are (or used to be, before a long list of insults) America’s closest allies, Trump argues that any foreign Islamic State fighter who survived and wants to wreak havoc at home is much more likely to be a European than an American. 

This approach might appeal to Trump’s “America First” constituency, as well as those weary of spending billions to hunt for terrorists behind every tree. But it misses the point. And Biden might be missing it, too, if he was arguing that the Islamic State is going to dispatch members to attack the United States. 

If the Islamic State can even partially rebuild, it will gain prestige from having withstood a U.S.-led effort to eradicate it. Unlike al-Qaeda, directly organizing attacks never has been its main threat to the West. Instead it would inspire lone individuals or small groups to launch attacks in their own countries. Experience has shown how hard that is to stop. 

In Iraq and Syria alone, the Islamic State has plenty of blood on its hands. Its attack on the Yazidi community in northern Iraq is widely regarded as an act of genocide. It is responsible for the beheadings of Americans, French, British citizens and many others.

It also has inspired deadly attacks in many other countries, most of them in the Middle East. But in France, the group claimed responsibility for the shooting and bombing spree in Paris in November 2015 that killed 130 people, and an attack by a follower who drove a truck through a Bastille Day crowd in Nice the next year, killing 86. In Britain, it claimed responsibility for a 2017 bombing at an Ariana Grande concert in Manchester that killed 22 people, and an attack on London Bridge that killed eight.

In the United States, attacks in San Bernardino, California, which killed 14 people, and the Pulse nightclub in Orlando, Florida, which killed 49, were inspired by the Islamic State.

It’s not really a coincidence that those attacks happened at least a couple of years ago. Intelligence and police work have improved, but the Islamic State also has been on the defensive. Its biggest prize, the northern Iraqi city of Mosul, fell in July 2017 after months of bitter fighting; Raqqa, its self-proclaimed capital in Syria, three months later. Its last stronghold in Syria fell six months ago.

Tens of thousands of its fighters have been killed. But thousands of others remain, and experts say they’ve regrouped to fight a rural insurgency. The International Crisis Group says the Islamic State has “taken refuge in some of Iraq’s most forbidding terrain, including mountains and caves, remote desert, orchards, river groves and islands.” Most fighters there now are Iraqis. It’s hard for foreigners to survive without an intimate knowledge of the population and landscape.

In Syria, former Defense Department officials Brian Katz and Michael Carpenter say that even before Trump’s move, the group had been carrying out guerrilla attacks, and gathering intelligence for assassinations, suicide attacks and kidnappings – “grooming the battlefield militarily, politically and psychologically so that it can go on the offensive as soon as the U.S.-led coalition withdraws.”

Groups affiliated with al-Qaeda are undergoing a similar process in western Syria in somewhat different circumstances, they say. 

It would be nearly impossible to eliminate the Islamic State entirely. With the United States out of the picture, that won’t be the main goal for anyone left in Syria. So the Islamic State will gain time and space to regroup. It will inspire followers to try to kill more innocent people in Paris, or Orlando, or another city. Maybe that’s a chance we’re willing to take; maybe not. But that’s a question we need to ask.