WASHINGTON — President Obama embraced drone strikes in his first term, and the targeted killing of suspected terrorists has come to define his presidency.

But lost in the contentious debate over the legality, morality and effectiveness of a novel weapon is the fact that the number of strikes has actually been in decline. Strikes in Pakistan peaked in 2010 and have fallen sharply since then; their pace in Yemen has slowed to half of last year’s rate; and no strike has been reported in Somalia for more than a year.

In a long-awaited address on Thursday at the National Defense University, Mr. Obama will make his most ambitious attempt to date to lay out his justification for the strikes and what they have achieved. He may follow up on public promises, including one he made in his State of the Union speech in February to define a “legal architecture” for choosing targets, possibly shifting more strikes from the C.I.A. to the military; explain how he believes that presidents should be “reined in” in their exercise of lethal power; and take steps to make a program veiled in secrecy more transparent.

Previewing the speech last weekend, an administration official speaking on the condition of anonymity said Mr. Obama would also “review our detention policy and efforts to close the detention facility at Guantánamo Bay; and he will frame the future of our efforts against Al Qaeda, its affiliates and adherents.” Some Obama supporters have urged him to use the occasion to announce that part of a 6,000-page Senate study of the C.I.A.'s former interrogation program will be declassified and made public.