U.S. commanders in Afghanistan say they expect the Taliban to launch a winter offensive, a move that could bring bloodshed during a time of year that historically has been relatively peaceful.

Since the U.S. invasion in 2001, violence in Afghanistan has usually tracked the country's seasons, decreasing during the harsh winter months and then resuming in the spring. This year is shaping up differently, with the U.S. picking up indications that militants who normally spend the winter months training in Pakistan are instead preparing...