Twenty years after Sept. 11, 2001, attacks set off the global war on terrorism, now that war is evolving.

The U.S. withdrawal from Afghanistan is ushering in a new era as the Biden administration seeks to balance the continued need to keep terrorist threats in check with both a war-weary public and newer global challenges such as competition with China.

As a result, the so-called forever wars may not be ending any time soon, but the way they are being fought is changing as the United States shifts to a greater reliance on “over the horizon” forces and individual strikes over broader military conflict.

“What we’re seeing is an evolution, and it’s an evolution in how it is the United States has decided to contend with the terrorist threat and how the forever wars are conducted,” said Becca Wasser, a fellow at the Center for a New American Security.

“This might be the next part of this broader evolution to see what really works when you’re removed even further,” she added. “It’s definitely going to be sort of a period of trial and error. And, unfortunately, when it comes down to counterterrorism, there’s not a lot of room for trial and error, especially if there are potential risks to the U.S. homeland. I think over time, the United States will find a formula that works. But I think there’s going to be a lot of back-and-forth before Washington can really get to that point.”

President Biden’s decision to fully withdraw from Afghanistan, a mission that was completed at the end of August, capped an era of U.S. history that began 20 years ago when al Qaeda hijackers crashed planes into the World Trade Center, the Pentagon and a field in Pennsylvania, killing nearly 3,000 people.

Shortly after the attacks, the United States invaded Afghanistan. In the ensuing 20 years, U.S. military counterterrorism operations spread around the globe, with troops targeting al Qaeda, its affiliates and, later, ISIS everywhere from Iraq and Syria to Libya to Yemen to Somalia to the Philippines.

The 2001 authorization for the use of military force passed in response to the 9/11 attacks has been invoked for military operations in at least 19 countries, according to a 2018 Congressional Research Service report.

U.S. concerns over the last two decades have also evolved, including efforts to pivot foreign policy away from the war on terrorism toward so-called great power competition with Russia and China, as well as a rising threat from domestic, white supremacist terrorists.

“Unlike on 9/11, the U.S. is in a different position today on the domestic terrorism side,” said Seth Jones, director of the transnational threats project at the Center for Strategic and International Studies. “We do see definitely more concerns today from domestic terrorism. I think that’s probably going to continue for the near term.”

But the terrorist threats abroad remain.

“One of the fallacies of the GWOT era is that this was something that could come to a conclusion, that all of a sudden the terrorist threat was going to be eliminated. And, unfortunately, that’s not how terrorism works,” Wasser said, using an acronym for the global war on terrorism. “It’s not a single enemy that you can quell and quash and isn’t going to rise back.”

Though Biden withdrew from Afghanistan, there are no plans at the moment to pull out the 2,500 U.S. troops in Iraq or the 900 in Syria helping local forces prevent an ISIS resurgence, though Washington and Baghdad recently rebranded the Iraq operation as purely advisory.

After a pause at the beginning of the Biden administration, U.S. strikes against Somalia’s al Qaeda affiliate have recently picked back up, with American forces conducting at least four strikes against al Shabaab in July and August.

And in Afghanistan, with the Taliban back in power, U.S. officials are also on the watch for a resurgent al Qaeda.

“The whole community is kind of watching to see what happens and whether or not al Qaeda has the ability to regenerate in Afghanistan,” Defense Secretary Lloyd Austin told reporters traveling with him in Kuwait this week.

“The nature of al Qaeda and ISIS-K is they will always attempt to find space to grow and regenerate, whether it’s there, whether it’s in Somalia, whether it’s in any ungoverned space,” Austin added, referring to the Afghan branch of ISIS.

Over the past 20 years, terrorist threats have come in “waves and reversal,” Jones argued as he expressed concern about a coming “fifth wave.”

“The campaign has gone through a series of waves and then reversals,” he said. “So waves meaning there’s been a surge in terrorist activity. And then it’s been reversed by either an effective counter terrorist campaign or bad terrorist activity, poorly run, poorly organized. And so I think we’re coming potentially now to the tail end of a reversal where al Qaeda and the Islamic State have actually suffered serious damage. They lost control of significant territory, they’ve been on the run, and the U.S. is now releasing pressure.”

Even though there are no U.S. troops on the ground, Biden has pledged to continue targeting terrorists in Afghanistan through “over the horizon” forces — or drones, manned aircraft and other assets based beyond Afghanistan’s borders.

The U.S. has already carried out two over-the-horizon drone strikes after last month’s attack at the Kabul airport that killed 13 U.S. troops. But officials have not identified the supposed “high profile” targets of the first strike, and reports on the ground say at least 10 civilians were killed in the second strike, with a New York Times visual investigation saying the target was an aid worker.

Experts and some military and intelligence officials have warned that over-the-horizon operations in Afghanistan will be difficult without a presence on the ground collecting intelligence. Further, no basing agreements with Afghanistan’s neighbors means flying from long distances, most likely from the Gulf region.

“Over the horizon is a crazy concept for Afghanistan with no bases, with no allies and partners on the ground, with no intelligence infrastructure,” Jones said.

Barry Pavel, director of the Atlantic Council’s Scowcroft Center for Strategy and Security, similarly argued relying on over-the-horizon forces will make it more difficult to track terrorist threats.

“This ‘over the horizon,’ wonderful bumper sticker, it’s pretty hard to do, and it takes more resources even to do it minimally because they have to fly from longer distances, you have to maintain the orbits. It’s just going to be harder,” he said.

But as U.S. counterterrorism efforts evolve, Pavel also sees an opportunity to do something the United States has done a “terrible job” at in the last 20 years: diplomacy, development and other efforts to address “the sources of alienation that lead people, in particular in the Middle East, to choose this as a very dangerous profession.”

“We’re gonna have to keep doing this until we do a better job of drying up the sources of why people get motivated to do this kind of stuff, and that’s a long term proposition that I don’t think we’re making very good progress on,” he said. “The Biden administration has an opportunity here, especially as an administration that wants to reduce the emphasis on military instruments.”