President Trump reportedly decided yesterday to keep U.S. troops on the ground in Syria a while longer, walking back his declaration on Tuesday that it's time to bring those troops home because "we've had a tremendous military success against ISIS."

Why it matters: Ending the U.S. presence in Syria is a bad idea. American troops have provided valuable support and intelligence to local Kurdish forces fighting ISIS and have at times engaged in direct combat. A U.S. withdrawal would constitute a strategic victory for Bashar al-Assad’s abhorrent regime, for its patrons Iran and Russia and for ISIS itself, allowing the group a chance to recoup.

A small U.S. military presence is still needed to train and advise Syrian Kurds and on rare occasions to carry out operational missions against vulnerable, high-value targets. This is nothing like what was attempted in Iraq and Afghanistan — which was nation-building, not counterterrorism — and Trump is right to argue that the costs incurred in those countries far exceeded any benefits.

What's next: The U.S. needs an endurance and not an exit strategy. The commitment would continue as long as ISIS and similar groups are present or until local forces are willing and able to contend with them on their own.

This is not a conventional fight against a conventional adversary. There will be no surrender, no ceremony on a battleship Missouri or at a courthouse in Appomattox or anywhere else. The struggle against terrorists — people prepared both to kill innocents and to die for a political cause — is open-ended. It is also essential, given the toll that another 9/11 would exact.

The bottom line: Success should be defined as limiting the problem of terrorism, not ending it. Trump needs to accept this — and level with the American people about what it will require.

Richard Haass is president of the Council on Foreign Relations and the author of “A World in Disarray.”