The vast majority of bombing runs, including the weekend strike near Mosul, Iraq’s second-largest city, are now searching for targets of opportunity, such as checkpoints, artillery pieces and combat vehicles in the open. But only one of every four strike missions — some 800 of 3,200 — dropped its weapons, according to the military’s Central Command…

In neither country are American commandos conducting raids on militant camps or safe houses, operations that in Afghanistan and in the Iraq war generated a continuous trove of information for additional missions.

Airstrikes have also been constrained by a serious concern about civilian casualties, particularly in western Iraq. Commanders fear such casualties could alienate Sunni tribesmen, whose support is critical to ousting the militants, as well as Sunni Arab countries that are part of the American-led coalition. Another challenge is weather, as sandstorms have thwarted many surveillance missions needed to identify targets.