President Trump has ordered the withdrawal of 2,000 American troops from Syria, bringing a sudden end to a military campaign that largely vanquished the Islamic State but ceding a strategically vital country to Russia and Iran.

In overruling his generals and civilian advisers, Mr. Trump fulfilled his frequently expressed desire to bring home American forces.

The abrupt, chaotic nature of the move immediately raised questions about how Mr. Trump will follow through with the full withdrawal.

Even after the president’s announcement, officials said, the Pentagon and State Department continued to try to talk him out of it.

“We have won against ISIS,” Mr. Trump declared in a video posted Wednesday evening on Twitter, adding, “Our boys, our young women, our men — they’re all coming back, and they’re coming back now.”

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