In 2009, an American drone fired a missile that ended the bloody career of Baitullah Mehsud on a rooftop in Pakistan. He was the Taliban's chief terrorist in that nation. Mehsud's mistake: He showed his face for a few minutes, long enough for the unmanned aircraft to get a fix. He wouldn't be the last terrorist to make that fatal mistake.

President Obama has aggressively championed drone strikes as a significant part of America's counterterrorism efforts around the globe, just as his predecessor, George W. Bush, did. The campaign has been extraordinarily effective, targeting militants across Pakistan, Afghanistan, Yemen and parts of Africa. It has helped cripple Al-Qaida leadership.

Many people, including some members of Congress, are troubled by the risk of civilian casualties and the largely secret rules by which drone controllers select and strike targets. The scope of drone warfare took center stage Thursday in the Senate Intelligence Committee hearing on the nomination of John Brennan to run the CIA. The hearing was frequently interrupted by protesters who accused Brennan of war crimes.

Brennan, who has played a pivotal role in managing drone attacks as the White House's counterterrorism adviser, offered a compelling defense of the program as "legally grounded" and deeply rooted in strong intelligence. He said officials have used drones "judiciously" and only as a "last resort."

The secrecy around the program makes it difficult to judge how well the United States has done to avoid civilian casualties and select appropriate targets. But a recently leaked Justice Department memo provides confidence about the legal justification for the deployment of drones. "Targeting a member of an enemy force who poses an imminent threat of violent attack to the United States is ... a lawful act of national self defense," the memo says.

The memo restricts targets to "a senior operational leader of al-Qaida or an associated force." It requires that "an informed, high-level" American official determine that the targeted terrorist "poses an imminent threat of violent attack against the United States." It requires that officials determine that the target cannot be captured.

This is the future of national defense, more dependent on advanced technology and less dependent on risking boots on the ground.