Located in Arlington County, Virginia, the Pentagon has been the headquarters for the United States Department of Defense since 1943. The Pentagon's unique five-sided design houses the largest office space in the world. Find out more about this impressive structure in Pentagon. This is an AV2 media enhanced book. A unique book code printed on page 2 unlocks multimedia content. This book comes alive with video, audio, weblinks, slide shows, activities, hands-on experiments, and much more.