13 December

Western Front

 * Germans withdraw from the Yser Canal.

Eastern Front

 * Battle for Warsaw raging.
 * Galicia: Battle for Cracow ending owing to Austro-German offensive from Hungary.

Southern Front

 * Serbia: Rout of the Austrians completed.

Naval and Overseas Operations

 * British submarine HMS B11 enters the Dardanelles and sinks Turkish battleship Messudiya.