For nearly two decades, the fundamental premise of America’s counterterrorism strategy has been to prevent extremist groups from establishing territorial safe havens — spaces in which they train and plot, free from interference. With a prospective U.S. withdrawal from Afghanistan on the horizon, Gen. David Petraeus warned recently that a precipitate pullout could allow al-Qaida or the Islamic State to rebuild “a terrorist platform.” A growing number of experts have argued, however, that a preoccupation with safe havens is really an unhealthy obsession that produces unnecessary — and unending — military crusades.

So, do safe havens matter or not? The truth is that denying such sanctuaries is critical to effective counterterrorism, so long as some key caveats and distinctions are kept in mind.

Since the Sept. 11 attacks, the fight against terrorism has been, in substantial measure, a fight against safe havens. President George W. Bush declared that the United States would make no distinctions between terrorists and the nations that harbored them, the thinking being that access to territorial sanctuaries allowed groups like al-Qaida to organize, operate and grow.

Since then, this idea has driven U.S. military operations in Afghanistan, Iraq, Syria, Yemen, Somalia, Libya and elsewhere. Even U.S. President Donald Trump, who campaigned on a pledge to end America’s Middle Eastern wars, invoked the danger of safe havens in deciding to surge additional troops into Afghanistan in 2017. “A hasty withdrawal,” he said, “would create a vacuum that terrorists, including ISIS [Islamic State] and al-Qaida, would instantly fill.”

Critics of U.S. strategy, however, are skeptical. They argue that safe havens are largely irrelevant, given that much of the direct planning and preparation for 9/11 and other major attacks occurred in Western cities rather than failed states. They claim that efforts to deny terrorist safe havens are counterproductive, because those endeavors drain American resources and incite the hatred of Muslim populations. They thus conclude that the threat of safe havens can be downplayed, if not ignored.

These arguments are alluring, but wrong. We know, on the basis of more than 20 years of history, that safe havens matter enormously — although their impact sometimes manifests itself in complicated and indirect ways. Indeed, the major reason the safe havens issue remains so contested is that critics of U.S. strategy often miss the complex causal chain by which these sanctuaries enable terrorist attacks.

Consider the history of IS, which conquered vast swaths of Syria and Iraq in 2013-2014. It subsequently used that territory to undertake many activities that contributed to mass-casualty terrorist attacks in other countries: producing print and digital propaganda that radicalized admirers and attracted tens of thousands of foreign fighters; providing training and instruction to new recruits; creating an international operations apparatus in northern Syria; and controlling valuable resources that funded a ruthless jihadist army and underwrote the apparatus of terror.

Several of the perpetrators of horrific assaults on Paris in November 2015 and Brussels in March 2016 had fought with and received training from IS in Syria, for instance. It also stands to reason that IS’s ability to inspire “lone wolf” attackers, as well as its successes in winning the loyalties of terrorist groups from Afghanistan to Nigeria, can be partially attributed to the ideological fervor and global prominence its territorial conquests helped evoke. (By the same token, the crushing of IS’s territorial caliphate will presumably reduce, if only gradually, the organization’s ability to inspire such attacks and loyalties.)

All of this was in addition to the gross human rights violations IS perpetrated within its territory, and the way it used the caliphate as a platform to destabilize a large portion of the Middle East. In other words, the IS safe haven promoted terrorist attacks directly, by providing resources and a base of operations, and indirectly, by attaining the prestige, ideological appeal and global reach that inspired admirers to shed blood on the caliphate’s behalf.

Or consider the 9/11 attacks. Yes, the detailed planning of those attacks occurred largely outside of Afghanistan. But the 9/11 attackers were drawing on the inspiration, resources and leadership of an organization that had surged to prominence by establishing territorial strongholds in Sudan and then Afghanistan, and using those areas as platforms from which to oversee escalating attacks against U.S. interests in the Middle East, Africa and ultimately the American homeland.

The same pattern holds true for one of al-Qaida’s most virulent offshoots, al-Qaida in the Arabian Peninsula. That group trained, in its Yemeni haven, at least one of the men who attacked the offices of the satirical magazine Charlie Hebdo in Paris in early 2015. It also commanded or inspired other attacks, including the failed “underwear bomber” attempt on a U.S. airliner in 2009. Other examples abound: Al Shabaab has used Somali territory to build the capabilities needed to carry out major assaults such as the Westgate mall attack in Kenya in 2013.

In sum, safe havens have a multiplier effect on terrorists’ aspirations and capabilities, giving them a variety of tools that can be weaponized to sow deadly violence.

No responsible counterterrorism strategy would focus solely on safe havens, to the exclusion of diplomatic and law enforcement tools, economic statecraft and other measures. And it is possible, as several terrorist groups have shown, to carry out attacks without controlling a sanctuary. Yet no responsible counterterrorism strategy can ignore the danger these spaces pose.

Does this mean that the U.S. should pay whatever it costs, for as long as it takes, to deny every safe haven to every terrorist group? Of course not. Not all safe havens are created equal: Those controlled by groups whose ambitions are largely confined to their home countries, such as Boko Haram in Nigeria, are less threatening than those controlled by groups with global ambitions.

The U.S. can be somewhat more tolerant of the less-threatening sanctuaries, and it can attack them primarily through indirect means such as support for the armed forces of friendly governments. Even in higher-priority theaters, cost is a limiting factor: If it takes 150,000 troops over a decade or more to prevent Qaida and IS from operating freely in Afghanistan, then that price is not worth paying.

Yet preventing the most dangerous groups from establishing secure territorial footholds is worth a significantly lower investment of resources, in the form of direct counterterrorism strikes and assistance to local security forces. The goal of this investment should not be to “win” the war on terrorism, but simply to suppress the worst aspects of the threat until a more stable governing model emerges in the Middle East.

Reasonable people can debate the proper level of this investment — 5,000 versus 15,000 troops in Afghanistan; the right size for a stay-behind force in Iraq and Syria; and so on. But we should remember that there are dangers associated with underinvesting as well as in overinvesting. Because if a major terrorist attack occurs in a vacuum created by inattention or withdrawal — as intelligence agencies have reportedly predicted might happen in the U.S. following a withdrawal from Afghanistan — the political pressure for a significantly larger, and less sustainable, military intervention might be irresistible.

So critics of U.S. counterterrorism strategy are right, in a sense: Washington does need to meter its military involvement in the Middle East. Yet the best way of doing that, over the long term, is to continue taking the threat of safe havens seriously.

Bloomberg Opinion columnist Hal Brands is a professor at Johns Hopkins University’s School of Advanced International Studies, and a scholar at the American Enterprise Institute. Most recently, he is the co-author of “The Lessons of Tragedy: Statecraft and World Order.”

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