LONDON — Revelations of new high-level losses among Al Qaeda’s top leadership in Pakistan’s tribal belt have underscored how years of American drone strikes have diminished and dispersed the militant group’s upper ranks and forced them to cede prominence and influence to more aggressive offshoots in Yemen and Somalia.

While the C.I.A. drone strike that killed two Western hostages has led to intense criticism of the drone program and potentially to a reassessment of it, the American successes over the years in targeting and killing senior Qaeda operatives in their home base have left the militant group’s leadership facing difficult choices, counterterrorism officials and analysts say.

That process of attrition has been accelerated by the emergence of the Islamic State, whose arresting brutality and superior propaganda have sucked up funding and recruits. In the tribal belt, a Pakistani military drive that started last summer has forced Qaeda commanders into ever more remote areas like the Shawal Valley, where two of them were killed alongside an American hostage, Warren Weinstein, and an Italian, Giovanni Lo Porto, on Jan. 15.

Even the death of Mr. Weinstein, a prized hostage whom Al Qaeda had long sought to exchange for prisoners or money, is emblematic of the state of siege. Whereas in Syria, the Islamic State has turned hostage execution into a macabre propaganda spectacle, Al Qaeda has seen any dividend from its captives snatched away, albeit inadvertently, by its American foes.