BAGHDAD — From the battlefield near Baiji, an Islamic State jihadist fired a heat-seeking missile and blew an Iraqi Army Mi-35M attack helicopter out of the sky this month, killing its two crew members. Since much of the most difficult fighting in the coming months is expected to unfold in the towns and cities of Anbar, American generals may be inclined to order more Apaches to support Iraqi ground troops. They may also make greater use of AC-130 gunships, a lumbering, propeller-driven plane bristling with cannons that circles at altitudes at the outer limits of some shoulder-fired missiles. Since at...