Gen. Joseph Votel told the House Armed Services Committee that unless the extremist group and its ideology are handled properly, IS will sow the seeds of future violent extremism.
"What we are seeing now is not the surrender of ISIS as an organization but a calculated decision to preserve the safety of their families and preservation of their capabilities," said Votel, adding that the insurgents are going to ground in remote areas, "waiting for the right time to resurge."
Votel said IS now holds less than a single square mile, a retreat that he called "a monumental military accomplishment." But he said the fight against IS and violent extremism is far from over.
A stubborn group of militants has kept that sliver of land in the Middle Euphrates River Valley for weeks, and many militants have escaped and scattered to other areas of the country where they are now in hiding.
In time, said Votel, IS will emerge more as an insurgency, and the group will focus on smaller, low-level attacks, assassinations and other violence that could disrupt efforts to bring stability to the country. The U.S. and its allies, he said, will have to keep up the pressure on the group and continue to train and advise the Syrian Democratic Forces and other partners on the ground.
Lawmakers peppered Votel with questions about Trump's sudden decision to withdraw from Syria — which was abruptly announced in a tweet. Votel and Katie Wheelbarger, the acting assistant defense secretary for international affairs, said they did speak with coalition allies around the globe after the tweet to reassure them that the U.S. wasn't pulling out immediately.
Since then, Trump has agreed to keep about 400 U.S. troops in the country, including 200 in northeastern Syria and another 200 in al-Tanf in southern Syria. Votel said the key goal continues to be the defeat of IS.
"What seems to be driving the withdrawal is the president's split-second decision to send out a tweet saying we're going to get out of Syria," said Rep. Adam Smith, D-Wash., the panel's chairman. "The argument we want to make is that what's driving our military decisions is military necessity."
Votel said there is no pressure on him right now to meet a specific withdrawal date, and he said the U.S. is "making sure that we protect our forces, that we don't withdraw in a manner that increases the risk to our forces going through this."
House members also expressed deep concern about any potential for U.S. withdrawal from Afghanistan and the ongoing peace negotiations between the U.S. and the Taliban. Zalmay Khalilzad, the U.S. envoy leading the talks, has said there have been slow, steady steps forward in the meetings in Doha.
Votel was asked if the conditions in Afghanistan, where the Taliban continues to wage attacks against Afghan and coalition forces, are ripe for a U.S. withdrawal. He answered that "the political conditions where we are in the reconciliation right now don't merit that."
Lawmakers said the while they agree with the idea of negotiations for peace, they are not willing to trust the Taliban and believe the group and its al-Qaida links remain a threat to America.
The Taliban provided safe haven for al-Qaida militants who planned and orchestrated the Sept. 11, 2001 attack on the U.S.