Yet even as he moves the counterterrorism effort to a next stage, Mr. Obama plans to offer a robust defense of a continued role for targeted killings, a policy he has generally addressed only in passing or in interviews rather than in a comprehensive speech. A White House official said he “will discuss why the use of drone strikes is necessary, legal and just, while addressing the various issues raised by our use of targeted action.”

While Mr. Obama may not explicitly announce the shift in drones from the Central Intelligence Agency in his speech, since the agency’s operations remain formally classified, the change underscores a desire by the president and his advisers to balance them with other legal and diplomatic tools. The C.I.A., which has overseen the drone war in the tribal areas of Pakistan and elsewhere, will generally cede its role to the military after a six-month transition period as forces draw down in Afghanistan, officials said.

Drone strikes have already been decreasing in the past few years as targets have been killed and opposition has grown. John O. Brennan, the new C.I.A. director, has been eager to shift the agency more toward espionage, intelligence gathering and analysis and away from the paramilitary mission it has adopted since Sept. 11.

Senator Jack Reed of Rhode Island, a top Democrat on the Armed Services Committee, said that after more than a decade of war it was time to “rebalance” the missions of the Pentagon and C.I.A. “The policy is intended to refocus the activities of the intelligence community to collection, which is crucial,” he said.

But Mr. Obama’s moves may provoke criticism from some Republicans who say a law enforcement approach underestimates the continuing threat from terrorism. An aide to Representative Howard P. McKeon of California, chairman of the Armed Services Committee, said his boss would insist on “concrete answers” about what Mr. Obama planned to do with “terrorists who are too dangerous to be released” from Guantánamo.

In his letter to Congressional leaders, Mr. Holder confirmed that the administration had deliberately killed Anwar al-Awlaki, a radical Muslim cleric who died in a drone strike in September 2011 in Yemen. Mr. Holder also wrote that United States forces had killed three other Americans who “were not specifically targeted.”

The American involvement in Mr. Awlaki’s death has been widely reported, but the administration until now had refused to confirm it. Likewise, Mr. Holder confirmed the government’s role in the deaths of Samir Khan, who was killed in the same strike, and Mr. Awlaki’s son, Abdulrahman al-Awlaki, who died in another strike. The letter disclosed the death of a fourth American named Jude Kenan Mohammad but gave no further details.