This monograph examines building large unit staffs to include divisions, corps, and field armies during mass mobilization. Unlike generating company grade officers, creating leaders capable of operational art is more challenging under tight time constraints. To discover trends on how the Army has generated large unit staffs before, the monograph uses three historical periods, each beginning with a force size contraction and ending with the conclusion of a wartime mobilization. The findings include three key deductions that helped generate or expand large unit staffs including doctrine, officer education, and officer selection. First, doctrine allowed a common point of reference for staff officers to learn and bridge gaps in experience. Next, officer education was critical prior to and during war because an educated officer corps served as the primary trainers to new units preparing for deployment. Finally, successful units had commanders take an active role in identification and selection of personnel capable of adaptive and creative learning. Officers in large unit staffs required many intangible attributes that are difficult to screen based on paper qualifications alone.