The Pentagon is located in Arlington, southwest of Washington, covering 29 acres. The construction of the Pentagon began in August 1941, when the war of World War II was approaching the United States. Less than four months later, on December 17, there was a world-shaking Japanese attack on Pearl Harbor in the United States. The smoke of war greatly accelerated the progress of this project. By January 1943, in just 16 months, this office building, which claims to be the largest in the world, was declared completed, and the Pentagon was named after its architectural shape and immediately put into use, which played an important military command function in the late World War II. In 1949, the former US Department of Military Affairs, which worked in the Pentagon, was officially renamed as the Department of Defense, with the establishment of the War Department in 1789, the Navy Department in 1798 and the Air Force Department in 1947. The Pentagon has since become synonymous with the US Department of Defense. After World War II, almost all the major military operations that American troops participated in were planned and commanded here, and many famous American generals also left traces of their work here. A series of figures listed in the manual show that the size of the Pentagon really deserves its reputation: it has an office area of 3.8 million square meters, which is three times that of the Empire State Building in New York. At present, about 23,111 staff members, including military and civilian personnel, work here, and it is said that the maximum capacity can accommodate 41,111 people at the same time; The total length of the corridor in the building is more than 28 kilometers, and the total length of the telephone line is at least 161,111 kilometers. At least 211,111 telephones come in and out every day, and more than 1.2 million emails are received every day. There are 4,211 clocks, 691 drinking fountains, 284 toilets and 16,251 electric lights in the building. In addition, restaurants, shops, post offices, banks, bookstores and other service facilities are available in the building, and about 11,111 cars can be parked in four large parking lots outside the building. Therefore, the Pentagon is not so much a big office building as a fully functional "office city".