In the summer of 1985, Ronald Reagan, concerned about a spike in the number of international terrorist attacks from 1983 to 1985, delivered a speech on the subject before the American Bar Association. He offered a simple prescription: “There can be no place on earth left where it is safe for these monsters to rest or train or practice their cruel and deadly skills. We must act together, or unilaterally if necessary, to ensure that terrorists have no sanctuary anywhere.”

More than a decade later, in May, 1998, a few months before the embassy bombings in Kenya and Tanzania, as the threat grew from Osama bin Laden’s Al Qaeda network, which was then safely ensconced in Afghanistan under the protection of the Taliban, Bill Clinton spoke at the Naval Academy and promised “to work with other nations to eliminate terrorist sanctuaries overseas.”

Clinton’s top counterterrorism official, Richard Clarke, fashioned a proposal he called “Delenda” (Latin for “must be destroyed”) to “immediately eliminate any significant threat to Americans” by denying bin Laden his Afghan protectorate. Clarke remained at the White House at the beginning of the Bush Administration, and proposed much the same strategy in the months before the September 11th attacks.

But neither Reagan, nor Clinton, nor Bush made the “no sanctuary” policy a centerpiece of their national-security strategy. The idea of invading Afghanistan, absent a serious attack on American soil, seemed unreasonable. The authors of the 9/11 Commission Report wrote, in 2004, “Since we believe that both President Clinton and President Bush were genuinely concerned about the danger posed by al Qaeda, approaches involving more direct intervention against the sanctuary in Afghanistan apparently must have seemed—if they were considered at all—to be disproportionate to the threat.” The report added, “Insight for the future is thus not easy to apply in practice. It is hardest to mount a major effort while a problem still seems minor. Once the danger has fully materialized, evident to all, mobilizing action is easier—but it then may be too late.”

“No sanctuaries” became one of the central recommendations of the 9/11 Commission, which repeatedly warned that Al Qaeda, and groups like it, should never again be allowed to operate in a safe haven where they have “the operational space to gather and sift recruits,” especially in failed states. “If, for example, Iraq becomes a failed state, it will go to the top of the list of places that are breeding grounds for attacks against Americans at home,” the report noted. This was the year after the Bush Administration invaded Iraq, and three years into the war in Afghanistan, about which the report’s authors added, “Similarly, if we are paying insufficient attention to Afghanistan, the rule of the Taliban or warlords and narcotraffickers may reemerge and its countryside could once again offer refuge to al Qaeda, or its successor.” ...