During the first World War, Pepper found it difficult to stay "neutral in thought"; despite his German ancestry, he was an unabashed anglophile throughout his life. He later admitted "I began as a violent partisan of the Allies". He first participated in the Preparedness Movement in 1914, and during the next two summers was a member of the Provisional Training Regiment at Plattsburgh, New York. After America entered on the Allied side, Pepper served as chairman of the Pennsylvania Council of National Defense (1917–1919). His nephew Franklin Pepper, whom he called "dear to me as a son", died a war hero in France in 1918.