The Pentagon, seen at the bottom of this 1951 aerial photograph, is located in Arlington, Virginia, a few miles from the U.S. Capitol (upper left). Home to the Department of Defense, the enormous 6.24-million-square-foot concrete structure is the largest office building in the world, covering thirty-four acres. Built to house the burgeoning War Department on the eve of the United States' entry into World War II (1939–1945), it was constructed in just seventeen months. From the moment Secretary of War Henry L. Stimson and Army Chief of Staff General George C. Marshall moved into the building in November 1942, the Pentagon has served as the focal point of American military planning and operations. Vital decisions regarding the D-Day invasion of Europe and the development of the atomic bomb were made there during World War II. In subsequent years the Pentagon has been the setting for many more critical decisions, from the Cold War and the Vietnam War (1961–1975) to the wars in Iraq and Afghanistan. On September 11, 2001, terrorists flew a hijacked passenger jet into the building, killing 184 people and seriously damaging the structure but not shutting it down. With its iconic, five-sided shape, the Pentagon is one of the world's most recognizable buildings and it has come to serve as a symbol of American military strength.