President Barack Obama has nominated his top counterterrorism adviser, John Brennan, to be the next director of the CIA.

If there is an emerging Obama doctrine to deal with the threat from al Qaeda and its allies, it is clearly a rejection of the use of conventional military forces and a growing reliance instead on the use of drones and U.S. Special Operations Forces -- and Brennan has been central to Obama's policy.

In an April 30 speech at the Woodrow Wilson Center in Washington, Brennan laid out the rationale for the drone policy in more detail than any administration official had done publicly hitherto. He asserted that the drone strikes are legal both under the Authorization for Use of Military Force passed by Congress after the September 11 attacks and because, "There is nothing in international law that bans the use of remotely piloted aircraft for this purpose or that prohibits us from using lethal force against our enemies outside of an active battlefield, at least when the country involved consents or is unable or unwilling to take action against the threat."

This does not appear to be the view of Ben Emmerson, U.N. special rapporteur on counterterrorism and human rights, who announced plans at Harvard Law School in October to launch an investigation into U.S. drone attacks and the extent to which they cause civilian casualties.

One of Brennan's most significant legacies in the four years he has been the president's principal adviser on terrorism is the American drone campaign against al Qaeda and its allies in countries such as Pakistan and Yemen -- one that has shifted focus significantly in the past year or so.

On Thursday, a CIA drone strike in the South Waziristan tribal region of Pakistan killed Mullah Nazir, a leading Taliban commander. That strike garnered considerable coverage in media outlets around the world and by U.S.-based news organizations such as The New York Times and CNN.

The same day a CIA drone killed three suspected Al Qaeda in the Arabian Peninsula militants near the town of Rada'a in Yemen. There was scant media coverage of this attack.

Yet Thursday's dueling strikes in Pakistan and Yemen are emblematic of a quiet and largely unheralded shift in the way that the CIA conducts its operations.

The accompanying bar chart (click on it to enlarge) does a good job of representing this shift. The red bars are U.S. strikes in Pakistan and the gold bars are U.S. strikes in Yemen based on data collected from reliable news reports by the New America Foundation.

(Due to the difficulty of distinguishing between what may be a U.S. drone strike or an airstrike by the Yemeni air force, the true number of American drone attacks in Yemen could be even higher than is shown in this chart.)