The 91st Infantry Division (famously nicknamed as the "Wild West Division" with a "Fir Tree" as its Division insignia to symbolize its traditional home of the Far West) is an infantry division of the United States Army that fought in World War I and World War II. From 1946 until 2008, it was part of the United States Army Reserve. It was briefly inactivated from 2008 until 2010 when it was elevated back to a division size element as the 91st Training Division (Operations).

The United States Army Reserve (USAR) is a reserve force of the United States Army. Together, the Army Reserve and the Army National Guard constitute the Army element of the reserve components of the United States Armed Forces.