WASHINGTON—In a move that significantly restricts the eligibility of thousands of American troops to fight for their country on the front lines, senior U.S. military officials announced Wednesday that all men with girl names would now be forbidden from serving in combat roles.

The ban, which goes into effect immediately, prohibits male personnel with clearly feminine names like Jamie, Sandy, and Alexis from serving in all artillery, infantry, and armored units. According to a Defense Department memo, the military is less effective as a fighting force when it deploys men named Francis, Sloan, Carol, or Loren in active conflict zones.