The 40th Infantry Division is a modular division of the United States Army. Following the army's modularization the division has become a four-brigade combat team with National Guardsmen from throughout the Pacific/Western United States and Oceania. Its division headquarters is located at Los Alamitos Joint Forces Training Base in Los Alamitos, California.

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.