The Pentagon is the headquarters of the United States Department of Defense, located in Arlington, Virginia. It is a large, five-sided building that serves as the central command for the U.S. military. Constructed during World War II, it is one of the largest office buildings in the world, housing approximately 23,000 military and civilian employees. The building is named for its unique shape and covers about 6.5 million square feet. The Pentagon is not only a symbol of U.S. military power but also a site of significant historical events, including the September 11, 2001 terrorist attacks, which targeted the building directly.