WASHINGTON — Now that the Islamic State has been driven from its last sliver of territory in Syria, hundreds of American troops — not just their equipment — are leaving the war zone, just as President Trump ordered in December.

But in the latest twist to the on-again, off-again American withdrawal, the Pentagon plans to cut its combat force in northeastern Syria roughly in half by early May, or to about 1,000 troops, and then pause, American officials said on Friday.

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The military will then assess conditions on the ground and reduce the number of forces every six months or so, until it reaches the 400 troops in Syria that Mr. Trump approved in February. They will include 200 in a multinational force in the northeast and another 200 at a small outpost in southeastern Syria, where they will seek to counter Iran’s influence throughout the country.

The pause is another important shift from Mr. Trump’s initial order to pull out all 2,000 American troops from Syria immediately, and has not been previously reported.