Across the Potomac River from Washington, D.C., stands the Pentagon, headquarters of the U.S. Department of Defense and a five-sided concrete and steel symbol of America’s military strength. Encompassing more than 6 million square feet of floor space, the Pentagon ranks among the largest office buildings in the world. On September 11, 2001, shortly after terrorists flew hijacked commercial airliners into New York City’s World Trade Center, another hijacked plane struck the Pentagon, killing 189 people and damaging roughly one-third of the building. The extensive post-9/11 repairs and renovations of the Pentagon were completed in exactly one year, a hectic pace matched only by that of the building’s original design and construction 60 years earlier.