WASHINGTON—A day after a contested decision to pull American military forces from Syria, officials said Thursday that President Trump has ordered the start of a reduction of American forces in Afghanistan.

More than 7,000 American troops will begin to return home from Afghanistan in the coming weeks, a U.S. official said. The move will come as the first stage of a phased drawdown and the start of a conclusion to the 17-year war that officials say could take at least many months. There now are more than 14,000 U.S. troops in Afghanistan.

Mr. Trump announced Wednesday that he would pull all of the more than 2,000 American troops from Syria.

Taken together, the Syria withdrawal and the likely Afghan drawdown represent a dramatic shift in the U.S. approach to military engagement in hot spots around the world, reflecting Mr. Trump’s aversion to long-running military entanglements with their high costs and American casualties.

“I think it shows how serious the president is about wanting to come out of conflicts,” a senior U.S. official said of how the Syria decision affects his thinking on Afghanistan. “I think he wants to see viable options about how to bring conflicts to a close.”