U.S. troops are beginning the process of leaving Afghanistan, after almost 20 years of fighting. Announcing his decision to complete the U.S. withdrawal by September, President Biden declared: “I believed that our presence in Afghanistan should be focused on the reason we went in the first place: to ensure Afghanistan would not be used as a base from which to attack our homeland again. We did that. We accomplished that objective.”

But al-Qaeda — which, after 9/11, provided the U.S. rationale for invading Afghanistan — still has 400 to 600 members fighting with the Taliban, according to U.N. Security Council estimates. In a recent interview, al-Qaeda operatives promised “war against the U.S. will be continuing on all other fronts.” Citing concerns about an al-Qaeda resurgence, several members of Congress blasted Biden’s decision. More quietly, many of the president’s military advisers also opposed the U.S. move to withdraw.

Critics of Biden’s decision warn that al-Qaeda remains strong — and that, if U.S. troops depart, the Taliban will allow it a haven, U.S. counterterrorism pressure will decline, the Afghan government will struggle and attacks on the United States from Afghanistan will occur. How valid are each of these concerns?

How strong is al-Qaeda in 2021?

Although al-Qaeda has fighters in Afghanistan, its ability to launch international terrorist attacks from there and from Pakistan, where the core organization has been based for almost 20 years, is limited. Al-Qaeda core members haven’t successfully attacked the U.S. homeland since 9/11, despite numerous attempts, and have also been ineffective against Europe in the past decade. Affiliate groups such as al-Qaeda in the Arabian Peninsula have conducted limited attacks, including a December 2019 attack that killed three people at a U.S. naval base in Florida, but they are not based in Afghanistan and Pakistan like the core. Indeed, in the past decade, al-Qaeda has localized more, relying heavily on affiliates to keep its name alive.

The United States has killed numerous al-Qaeda core leaders, and overall leader Ayman al-Zawahiri may be dead — the fact that we don’t know his status months after rumors of his death tells us something about his decreased relevance. Pressure from U.S. airstrikes and Special Forces raids have kept leaders on the run and have prevented the establishment of large-scale training camps akin to those that existed before 9/11.

And countries around the world remain focused on the al-Qaeda threat, in contrast with the pre-9/11 era. The global intelligence campaign has made it harder for the group to communicate, send its operatives to reconnoiter or raise money, or otherwise prepare to conduct attacks.

The Taliban lies, but it has reasons to restrain al-Qaeda

U.S. negotiators have pressed the Taliban for years to agree to prevent al-Qaeda from using Afghanistan as a haven for international terrorist attacks. The Taliban claims to have accepted this demand, and al-Qaeda members also claim they will honor this even as they attack the United States from other theaters. However, Taliban leaders have lied about the extent of their relationship with al-Qaeda in the past, casting doubt on denials about the future relationship between the two.

However, drawing the line at international attacks is logical for the Taliban. While it gave Osama bin Laden sanctuary, the Taliban opposed many of al-Qaeda’s attacks, including 9/11 — though it continued to cooperate with al-Qaeda despite being angry with the decision to attack the United States in 2001.

Since then, the Taliban has learned the cost of opposing the United States, and its leaders may recognize that keeping al-Qaeda personnel as fighters but drawing the line at anti-U.S. terrorist attacks satisfies the Taliban’s loyalty to its ally and desire for capable fighters while keeping Afghanistan out of U.S. crosshairs.

U.S. counterterrorism capabilities will diminish

The U.S. withdrawal will limit the U.S. ability to strike the al-Qaeda core in Pakistan and Afghanistan. The lack of U.S. troops is likely to hinder efforts to strike al-Qaeda directly — but also make it difficult to determine whether the Taliban is cheating on its commitment to halt al-Qaeda from conducting international attacks.

U.S. troops on bases in Afghanistan also protected intelligence assets and conducted raids against al-Qaeda, and the bases served as drone launching points. CIA Director William J. Burns recently testified that, after a withdrawal, “the U.S. government’s ability to collect and act on threats will diminish.”

However, the United States will be able to sustain at least some counterterrorism capacity outside Afghanistan and is already working on other basing options, including Qatar and Uzbekistan. These alternatives are farther away, however, which will make U.S. operations more difficult.

The Afghan government is likely to get weaker

“I am concerned about the Afghan military’s ability to hold on after we leave,” Gen. Kenneth “Frank” McKenzie, commander of U.S. Central Command, testified last week. This military weakness exists despite billions of U.S. dollars in aid, massive military training efforts and other support over the past two decades.

In the aftermath of a U.S. departure, the Afghan government’s ability to go after al-Qaeda may be limited at best. The operations of Afghan forces depended heavily on U.S. intelligence and military support. In addition, without heavy pressure or inducement from the United States, the Afghan government is more likely to focus on the Taliban and other immediate threats — it’s less likely to have the bandwidth to address small groups of terrorists setting up shop to conduct international attacks.

Pakistan, however, may have more of an incentive to separate out al-Qaeda from the Taliban, its longtime ally. Greater Taliban influence in Afghanistan is a victory for Pakistan, but Islamabad has its own Islamist militant challenge. An al-Qaeda attack from Afghanistan on the United States or Europe would put renewed pressure on the Taliban — an outcome Pakistan would oppose.

Taken together, these factors suggest the U.S. troop withdrawal will ease pressure on al-Qaeda, but the group is far from its pre-9/11 strength, and it faces many challenges. As a result, it is far from certain that international terrorist attacks are a likely consequence of the departure of U.S. forces from Afghanistan.

Daniel Byman (@dbyman) is a professor in the School of Foreign Service at Georgetown University and a senior fellow at the Brookings Institution. His latest book is “Road Warriors: Foreign Fighters in the Armies of Jihad” (Oxford University Press, 2019).