President Barack Obama's counter-terrorism adviser outlined on Wednesday an evolving military strategy that will increasingly rely on targeted, clandestine strikes rather than conventional military operations.

Obama's recently announced decision to accelerate the drawdown of U.S. forces in Afghanistan parallels a broader effort to rely increasingly on Special Operations forces and unmanned drone strikes. Senior Counterrorism Adviser John O. Brennan Wednesday described in a speech the need for targeted, surgical pressure on Al Qaeda, saying such strikes would effectively cripple the already reeling organization.

If we hit Al Qaeda hard enough and often enough, there will come a time when they simply can no longer replenish their ranks with the skilled leaders that they need to sustain their operations, Brennan said.

With Obama's approval, the Central Intelligence Agency recently launched an expanded campaign of covert strikes against militants in Yemen, where the collapse of civil order has both given the U.S. an opening and emboldened Islamic militants battling the government. A drone fired Wednesday on two senior leaders of Somalia's al-Shabab, a militant organization that officials say has ties to Al Qaeda and is planning strikes abroad. In his speech, Brennan cited the group as a future target.

As the al-Qaeda core has weakened under our unyielding pressure, it has looked increasingly to these other groups and individuals to take up its cause, including its goal of striking the United States, Brennan said. From the territory it controls in Somalia, he said, al-Shabab continues to call for strikes against the United States.

Such strikes are sanctioned under an Authorization for Use of Military Force, passed in the days following Sept. 11, 2011, that gives the president broad powers to deploy force against individuals responsible for Sept. 11 or people who harbor those responsible. The attacks in Somalia could stretch the limits of that mandate.

The further and further we get away from 9/11, with the core of Al Qaeda decimated and Bin Laden dead, the way the original Authorization for Use of Military Force is drafted, it gets more and more tenuous, Cato Institute Scholar Gene Healy said. They are treating it like a permanent delegation by the Congress to the president of a generalized war power to launch attacks worldwide.

The increasing emphasis on such strikes is also telegraphed in Obama's appointment of U.S. Army Gen. David Petraeus, currently the commander of U.S. forces in Afghanistan, to lead the CIA. The move underscores the CIA taking on a greater role in counter-terrorism activities as clandestine strikes assume a more prominent role. At his Senate confirmation hearings, Petraeus referenced a broad U.S. network of interlocking counterterrorism operations.

One of the major developments since 9/11 has been the establishment of this network, in many cases led by the Joint Special Operations Command of the military, but with very, very good partnering with elements of the Central Intelligence Agency, other elements of the intelligence community, Petraeus said.