A large military unit that can work independently or semi-independently on operations and is normally led by a Colonel. During World War II, a US Army infantry regiment was the primary infantry organizational unit. Identified by number, based on Army Classification System: Regular army regiments number 1-99, National Guard regiments numbered 100-299, Conscripted or Reserve regiments numbered in 300s. The regiment holds its own history and lineage. In 1957 the traditional regimental system was replaced by the Combat Arms Regimental System (or CARS, which used divisions, brigades and battalions as the primary units) and then in 1981 by the US Army Regimental System (USARS).