Dan de Luce notes that top military commanders expect that the war on ISIS could last decades:

But White House officials, and most members of Congress, are reluctant to speak publicly about how long the campaign may last, much to the frustration of military commanders. For members of both political parties, acknowledging that the war could drag on for another 10 to 20 years is politically risky, if not poisonous, and would require confronting difficult decisions about ordering troops into combat, budgets, and strategy.

When the administration first started its intervention against ISIS last year, officials talked about a war that would last at least three years, but that has proved to be overly optimistic. The war began as a “limited” and defensive action, but quickly morphed into an aggressive, open-ended, multi-year commitment that was never debated and was never put to a vote. The executive branch committed the U.S. to this policy on its own to show that it was “doing something” to combat ISIS, but it did so with no thought for the duration, cost, or difficulty of the war that it started. It has been able to get away with this because Congress is useless when it comes to overseeing the conduct of foreign wars, but it has still left the U.S. holding the bag for a costly conflict in Iraq and Syria with no discernible end. The U.S. is now stuck with a significant military mission that could conceivably last through the end of the next decade, and there has been no serious consideration of the risks or benefits in Congress or anywhere else. As has happened in the past, the U.S. has intervened first and failed to ask any of the relevant questions. This just drives home how unwise and unnecessary U.S. involvement has been from the start, and now that the U.S. is committed it will be extremely difficult to extricate U.S. forces from a war they should never have been fighting.