“The tempo is coming back,” Gen. Kenneth “Frank” McKenzie Jr., who heads U.S. Central Command, said during a visit to a constellation of tiny American outposts in eastern Syria.

American troops in Syria, like those in neighboring Iraq, shifted their focus from anti-extremist operations to force protection after a Jan. 3 strike in Baghdad killed Maj. Gen. Qasem Soleimani, Iran’s most influential military figure.