Finished in 1943, the Pentagon is a symbol of U.S. military might. It's also one of the places attacked on 9/11.
Construction on the Pentagon started on Sept. 11, 1941, in the midst of World War II. The five-sided design was chosen to accommodate existing roads around the building's original site near Arlington National Cemetery; a different site was ultimately picked, but the design was kept, according to the History Channel.
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