But there are fears the impending U.S. pullout will imperil those gains. Trump told coalition members meeting at the State Department that while "remnants" of the group were still dangerous, he was determined to bring U.S. troops home. He called on coalition members to step up and do their "fair share" in the fight against terrorism.
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Even as Trump and Secretary of State Mike Pompeo defended the withdrawal decision, which shocked U.S. allies and led to the resignations of Defense Secretary Jim Mattis and the U.S. envoy to the anti-ISIS coalition, Brett McGurk, some military leaders, renewed their concerns.
While the withdrawal would fulfill a Trump goal, top military officials have pushed back for months, arguing ISIS remains a threat and could regroup. U.S. policy had been to keep troops in place until the extremists are completely eradicated. Fears that ISIS fighters are making a strategic maneuver to lay low ahead of the U.S. pullout has fueled criticism that Trump telegraphed his military plans — the same thing he accused President Barack Obama of doing in Afghanistan.
Pompeo told the coalition that the planned withdrawal "is not a change in the mission" but a change in tactics against a group that should still be considered a menace.
"In this new era, local law enforcement and information sharing will be crucial, and our fight will not necessarily always be military-led," he said. Trump's announcement "is not the end of America's fight. The fight is one that we will continue to wage alongside of you."
He added: "America will continue to lead in giving those who would destroy us no quarter."
Yet senior military officials acknowledged to Congress on Wednesday that the pullout would complicate their efforts.
Owen West, the assistant secretary of defense for special operations, told the House Armed Services Committee that he shared Mattis' objections. West answered, "No, sir," when asked by a lawmaker if he thought Mattis was wrong to disagree with the withdrawal.
At the same hearing, Maj. Gen. James Hecker, vice director of operations for the Joint Chiefs of Staff said the withdrawal means "it is going to be difficult to keep up the pressure" on ISIS. "There will be a decrease in the amount of pressure that we will be able to apply," he said.
"The concern is if we move our forces out of Syria that that may take some pressure off of the ISIS forces in Syria," Hecker said. "So our mission is to try to figure out how we can continue to keep the pressure on in Syria without any boots on the ground."
Hecker said others would have to carry the burden once the U.S. left. He did not offer specifics.
In liberated areas across Syria and Iraq, ISIS sleeper cells are carrying out assassinations, setting up checkpoints and distributing fliers as they lay the groundwork for an insurgency that could gain strength as U.S. forces withdraw.
Activists who closely follow the conflict in Syria point to signs of a growing insurgency. Rami Abdurrahman, the head of the Britain-based Syrian Observatory for Human Rights, says IS still has 4,000 to 5,000 fighters, many likely hiding out in desert caves and mountains.
A United Nations report circulated Wednesday said Islamic State extremists "continue to pose the main and best-resourced international terrorist threat."