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 a reassessment of it, the American successes over the years in targeting and killing senior Qaeda operatives in their home base has left the militant group’s leadership facing difficult choices, counterterrorism officials and analysts say.