The Obama administration’s counterterrorism accomplishments are most apparent in what it has been able to dismantle, including CIA prisons and entire tiers of al-Qaeda’s leadership. But what the administration has assembled, hidden from public view, may be equally consequential.

In the space of three years, the administration has built an extensive apparatus for using drones to carry out targeted killings of suspected terrorists and stealth surveillance of other adversaries. The apparatus involves dozens of secret facilities, including two operational hubs on the East Coast, virtual Air Force­ ­cockpits in the Southwest and clandestine bases in at least six countries on two continents.

Other commanders in chief have presided over wars with far higher casualty counts. But no president has ever relied so extensively on the secret killing of individuals to advance the nation’s security goals.

The rapid expansion of the drone program has blurred long-standing boundaries between the CIA and the military. Lethal operations are increasingly assembled a la carte, piecing together personnel and equipment in ways that allow the White House to toggle between separate legal authorities that govern the use of lethal force.

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