The Army Reserve's 66th Cavalry Division was created from the perceived need for additional cavalry units. It numbered in succession of the Regular ArmyDivisions, which were not all active at its creation. Going into World War II, the U.S. Army Cavalry contained three Regular, four National Guard, and six organized reserve cavalry divisions as well as one independent cavalry brigade, the 56th from Texas.
The 66th Cavalry Division was dispersed across the United States. The division was primarily composed of personnel from Nebraska, Missouri, Utah, and North Dakota.