The Pentagon, located in Arlington, Virginia, is home to the Department of Defense and serves as military headquarters for the United States. 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), the headquarters was constructed in just seventeen months. From the moment Secretary of War Henry L. Stimson and Army Chief of Staff Generalmoved 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 at the Pentagon 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 Pentagon, killing 184 people and seriously damaging the building 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.