First, the airspace. So far, armed drones have been used either over countries that do not control their own airspace (Somalia, Mali, Afghanistan) or where the government has given the United States some degree of permission (Yemen, Pakistan). Those circumstances are rare. When the foe can actually defend itself, the use of armed drones is extraordinarily difficult and could constitute an act of war — one that could easily draw the United States into the heart of a conflict.

Drones are slow, noisy, fly at a low altitude, and require time to hover over a potential target before being used — they are basically sitting ducks. Syria has an air force and air defenses that could easily pick American drones out of the sky. The only real way to use them would be for the United States to first destroy Syrian planes and anti-aircraft batteries. But that would be no different from a full-scale intervention and would negate the tactical advantage of remote strikes. In other words, the conditions under which armed drones are effective as preventive weapons are limited. And the more drones are used for prevention and during war, the more state belligerents will take note of that fact, and will make sure that those conditions are never met on their own territory.

Second, the target. Using armed drones against the Syrian government’s enormous chemical weapons stockpiles would have risked causing the very release of deadly agents that the United States was trying to avoid. Drones are precise but not perfect. Like cruise missiles, their effectiveness mainly depends upon the quality of their targeting information. Worse, an imperfect attack could inadvertently give the Assad government political cover to use the weapons with impunity. Assad could blame the release of chemical weapons on a misfired U.S. drone strike. Since U.S. drones are deeply despised in the Middle East, that argument could enjoy wide hearing.