Top Pentagon leaders are warning that the long war is going to get even longer.
Defense Secretary Jim Mattis and Joint Chiefs Chairman Gen. Joseph Dunford told Senate leaders on Wednesday that even after ISIS is defeated in Syria and Iraq, U.S. troops will be stationed in the region for at least a few years afterward.
"I believe it's in our national interest that we keep Iraqi security forces in a position to keep our mutual enemies on their back foot," Mattis told members of the Senate Appropriations Committee on Defense.
"I don't see any reason to pull out again and face the same lesson," he added, referencing the removal of all U.S. forces from Iraq in 2011.
Though President Barack Obama in 2008 campaigned on a promise to pull troops out of Iraq, the move has been criticized by conservatives in the years since as helping fuel the rise of ISIS.
In 2014, as ISIS militants seized vast swaths of Iraq and Syria, one senior military officer told Business Insider the rise of the terror group in the wake of U.S. troop departures was inevitable.
"We said we won some success but this is reversible," the retired senior U.S. military officer said, on condition of anonymity. "So what we're seeing now is exactly what we forecasted."
So far, Mattis seems more comfortable placing troops closer to harm's way than his predecessor.
Though the Pentagon has long downplayed the role of U.S. ground troops in the fight against ISIS in Iraq and Syria, recent deployments of many more "boots on the ground" suggest they may be front-and-center in the coming months.
In addition to roughly 500 U.S. special operations forces, the military has sent conventional ground troops inside Syria, to include a contingent of Army Rangers, soldiers from the 82nd Airborne Division, and artillerymen from 1st Battalion, 4th Marines to provide fire support just 20-30 miles from Raqqa, the ISIS capital.
"The Iraqi security forces will need that kind of support for years to come," Dunford told the Senate committee.