The correct statement about the American Expeditionary Force (AEF) is:
O The AEF broke the stalemate in favor of the Allies.
The AEF played a crucial role in several key battles in 1918, including the Second Battle of the Marne and the Meuse-Argonne Offensive, contributing significantly to the breaking of the stalemate on the Western Front and leading to an eventual Allied victory in World War I.