Here’s the trickiest counterterrorism puzzle for U.S. policymakers: How do you stop al-Qaida from attacking the American homeland, without getting bogged down in protracted wars against insurgents?

One answer would be to establish deterrence in the long war against Islamic extremism, like the standoff that developed between the U.S. and the Soviet Union during the Cold War. The Soviets were obviously a far bigger adversary, but the basic logic is the same: Both sides have deadly weapons; but if you don’t shoot at us, we won’t shoot at you.

The Obama administration hasn’t declared any such neo-deterrence strategy, but you can see the outlines of one emerging in the administration’s unannounced targeting policy for armed Predator drones, which have been America’s deadliest weapon against al-Qaida.

The latest illustration of the drones’ precision power came in last Friday’s attack on Anwar al-Awlaki, a leader of al-Qaida in the Arabian Peninsula. The strike over northern Yemen, which followed days of silent surveillance, killed Awlaki and Samir Khan, the editor of an al-Qaida online magazine called “Inspire.” Both were U.S. citizens.

But in recent weeks a subtle limit has emerged in drone policy: Despite calls by some U.S. officials for drone attacks against the training camps of AQAP and al-Shabab, the al-Qaida affiliate in Somalia, neither has been targeted. That’s a deliberate policy decision — aimed partly at preventing the spread of a Taliban-style insurgency to new theaters, such as Yemen and Somalia.

The U.S. claims it has legal authority for such so-called “signature” strikes on training camps of al-Qaida affiliates, under both the congressional Authorization for Use of Military Force passed in September 2001 and the international law of self-defense. This broad legal authority for targeting was outlined by John Brennan, the White House counterterrorism chief, in a Sept. 16 speech at Harvard.

But as a matter of policy, Brennan and other top officials have decided (for now) against such strikes in the new battlegrounds, in part to prevent an ever-widening war that fosters the very Islamic insurgency we want to contain. Indeed, officials say there haven’t been any signature drone strikes against camps (as opposed to individuals) outside Pakistan’s tribal areas, where al-Qaida’s core leadership is based.

A senior administration official explains the policy this way: “If individuals target us, if they are in the chain of command for attacks against Americans,” then the U.S. will authorize “direct action” — putting such individuals on the “capture or kill” list that triggers a drone attack. But, the official cautions, “We don’t want to get involved in a domestic confrontation inside Yemen or Somalia, or increase anti-U.S. sentiment” in those places.

There is a deterrence formula implicit in this policy: So long as Somalia’s al-Shabab remains an insurgent movement fighting the Transitional Federal Government, the U.S. — while supporting the Somali authorities — won’t use drones. That weapon is reserved for those who directly threaten the U.S. “Al-Shabab can increase the danger to themselves if they attack us, or engage in actions designed to hurt our people,” says the official. “We can act in self-defense, but also to send the message that if you threaten us, you do so at your own peril.”

This calibrated approach has reassured key U.S. allies, such as Britain, which have large Muslim immigrant populations and were worried about the blow-back of U.S. campaigns against al-Qaida affiliates. “There was concern that it was a blanket approach,” concedes the U.S. official.

I asked the Obama administration official about the “due process” problem. On what basis does the U.S. issue what amounts to a death warrant against an American citizen, such as Awlaki? The official answered that reviews are conducted by an inter-agency committee of lawyers, and also by a committee of deputies of all key departments — and that strike orders require unanimous agreement. That’s only partly reassuring; a more rigorous legal process would be better, like the secret court that rules on foreign-intelligence search warrant requests.

What’s good about the evolving drone policy is that it recognizes the need for limits. We don’t have enough drones to kill all the enemies we will make if we turn the world into a free-fire zone. And there’s something important in the hint of a deterrence strategy: This is how wars end in the part of the world where al-Qaida arose — through a balance of mutual restraint that makes a de-facto truce possible, even between the most bitter enemies.