The Army not knowing what to do with an engineer who had never led troops or trained for combat suddenly elevated to a general of infantry, decided to assign Sibert, who had been working on canal projects in the Mid-West and advisory missions to China, to command the Pacific Coast's Coastal Artillery.

He was duly breveted to major general and deployed with the initial four regiments of the American Expeditionary Forces (AEF) which formed the 1st Division (nicknamed "The Big Red One") once in France.

The AEF's Commander-in-Chief (C-in-C), General John J. Pershing, a long serving cavalry 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 qualified general officers (he himself had only recently been promoted to his position) so Sibert was placed in charge of the 1st Division.

The AEF suffered a serious leadership problem throughout the final year of the war, as officers were rapidly promoted to positions with little or no experience.

The American Army was singularly unprepared for the war, and the strain of its rapid expansion created many personnel problems like Sibert's.

Actual promotion required Congressional approval, the number of positions limited by law, and was based solely on seniority.

[1] In October 1917, Pershing wrote an extensive letter to Secretary of War Newton D. Baker expressing his concerns about some of his generals, "I hope you will permit me to speak very frankly and quite confidentially, but I fear that we have some general officers who have neither the experience, the energy, nor the aggressive spirit to prepare their units or to handle them under battle conditions, as they exist today.

Pershing was dissatisfied with the Division's progress and elevated Brigadier General Robert Lee Bullard, a true line officer, to replace Sibert.

[6] Sibert returned to the United States in January 1918 where he became the commanding general of the Army Corps of Engineers Southeastern Department located at Charleston, South Carolina.

When the War Department created the Chemical Warfare Service (CWS) later that spring, Pershing was asked to name a general officer to head it.

She died 15 months later and in 1922 Sibert married Evelyn Clyne Bairnsfather of Edinburgh, Scotland who remained his wife until his death on October 16, 1935, in Bowling Green.