as his
aide-de-camp, 1915. Mann left the Militia Bureau in 1917 to organize and train a division for overseas serviceand was succeeded by
Jesse McI. Carter. He became the first commander of the
42nd Division, nicknamed the Rainbow Division, which was composed of
National Guard units from 26 states and the
District of Columbia. The 42nd Division's activation was important in the development of the National Guard because it was the first time National Guard units from multiple states were organized together and it was the first time smaller Guard units were formed into a division. Mann was within a year of mandatory retirement in November 1917, by which time he and his division had arrived in
St. Nazaire,
France, where it became part of the
American Expeditionary Forces (AEF). He began to suffer from poor health after the organization moved to its new location at
Vaucouleurs in
Lorraine, making chief of staff
Colonel Douglas MacArthur the
de facto commander. After Mann's superior, General
John J. Pershing,
Commander-in-Chief (C-in-C) of the AEF, made an on-site visit and observed Mann's infirmities, he cabled the
War Department requesting that Mann be relieved. Mann was relieved in command of the 42nd by Major General
Charles T. Menoher, a West Point classmate of Pershing's, and soon returned to the United States and commanded the
Department of the East until he retired from the army in July 1918. ==Post-military career==