Military leaders know that they are fighting a ruthless adversary that has adjusted and adapted its tactics as the United States and its partners have joined the fight over the past 18 months. The jihadists have lost about 25 percent of the territory they held in mid-2014, but they have devised innovative methods to compensate for their weakness.

Some examples illustrate the agility of Islamic State commanders: They have used tunnels and other concealment tactics to hide their movements; they have developed super-size car bombs, packing explosives in bulldozers and other heavy equipment and sending them in waves against targets; they have deployed small drones for reconnaissance and may be preparing armed drones; they have used chemical weapons, such as chlorine and mustard gas, on the battlefield and may expand use of such unconventional weapons.

U.S. commanders have learned how difficult it will be to create a Sunni force that can help clear and hold territory in Iraq and Syria that’s now controlled by the Islamic State. Sunni tribal leaders mistrust the United States and doubt U.S. staying power. U.S. efforts to avoid casualties and resist “boots on the ground” reinforce the sense that the United States is pursuing a strategy of containment, not victory.