Unfortunately for Sibert, when the United States entered World War I in April 1917, Brigadier General Sibert was one of the only senior infantry officers on active duty. He was duly breveted to major general and deployed with the initial four regiments of the American Expeditionary Force (AEF) which formed the 1st Infantry Division (nicknamed "The Big Red One") once in France. The AEF commander, General John Joseph Pershing, a long serving infantry officer famous for his exploits at San Juan Hill in the Spanish–American War, and recently in charge of the campaign against Pancho Villa, was short on general officers (he was himself only recently promoted to his position) so Sibert was placed in charge of the 1st Infantry Division.