Officials say the overhaul of Africa deployments will be followed by one in Latin America, and that drawdowns will happen in Iraq and Afghanistan, as has been expected.

The initiative reflects what has become the defining priority for Mr. Esper: moving away from 18 years of counterterrorism deployments in places troubled by militancy and insurgency where thousands of American troops cycle through in an attempt to maintain minimal stability but without much prospect of definitive solutions.

“We’ve begun a review process where I’m looking at every theater, understanding what the requirements are that we set out for, making sure we’re as efficient as possible with our forces,” Mr. Esper told reporters this month.

The details of planning for troop reductions in West Africa have been closely held in the Pentagon, and Congress has not been consulted, officials said. Mr. Esper’s decision could also affect other agencies: The military’s ability to provide swift backup for security at diplomatic and intelligence compounds in troubled parts of the world has been a heightened concern since the 2012 attack on outposts at Benghazi, Libya.

The primary mission of the American troops has been to train and assist West African security forces to try to suppress Islamist groups like Boko Haram and offshoots of Al Qaeda and the Islamic State. As part of that mission, four American soldiers were ambushed and killed two years ago while on patrol in Niger.