The Defense Department Headquarters is located in the Pentagon, an 8-letter word. The Pentagon is a large five-sided building located in Arlington County, Virginia, just outside of Washington D.C. It is the headquarters of the United States Department of Defense and is the world's largest office building by floor area. It was designed during World War II and completed in 1943. It is home to the offices of the Secretary of Defense, the Joint Chiefs of Staff, and the Defense Intelligence Agency.