After the attack on American diplomats in Benghazi last month, President Obama vowed to hunt down the killers and bring them to justice. There is a good chance that this means that they will be incinerated by missiles fired from drones. If so, the United States will have used drones to kill members of al-Qaida and affiliated groups in Afghanistan, Iraq, Pakistan, Somalia, Yemen, and Libya—six countries in just a few years. Mali may take its turn as the seventh. This startlingly fast spread of drone warfare signifies a revolution in foreign affairs. And, for good or for ill, in an unprecedented way it has transformed the U.S. presidency into the most powerful national office in at least half a century.