The 33rd Infantry Division was a formation of the U.S. Army National Guard between 1917 and 1968. Originally formed for service during World War I, the division fought along the Western Front during the Battle of Amiens, the Battle of Hamel, the Meuse-Argonne Offensive, at the Second Battle of the Somme, and at the Battle of Saint-Mihiel. It was re-formed during the inter-war period, and then later activated for service during World War II, seeing action against the Imperial Japanese Army in the Pacific. In the post war era, the division was reconstituted as an all-Illinois National Guard division. In the late 1960s, the division was reduced to a brigade-sized formation, and its lineage is currently perpetuated by the 33rd Infantry Brigade Combat Team.

The Army National Guard (ARNG), in conjunction with the Air National Guard, is an organized militia force and a federal military reserve force of the United States Army. They are simultaneously part of two different organizations: the ARNG of each state, most territories, and the District of Columbia, as well as the federal ARNG, as part of the National Guard as a whole. It is divided into subordinate units stationed in each state or insular area, responsible to their respective governors or other head-of-government.