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.
The Pentagon officially opened in January 1943 to house what was at that time called the War Department.
On Sept. 11, 2001, the Pentagon became the third target of terrorists who hijacked airliners.
The plane that crashed into the Pentagon penetrated the building's outer three rings, killing 125 people in the building and 89 people on the plane, including the five hijackers.
Construction to repair the Pentagon began quickly after the attacks. Dubbed "The Phoenix Project," the goal was to have most work completed by Sept. 11, 2012.
Workers started moving back into the rebuilt section of the Pentagon in August 2002, and the final tenants were back in that section in February 2003.
Work started on a memorial to those killed in the Sept. 11 attacks in 2006.
The National 9/11 Pentagon Memorial opened on the 2008 anniversary of the Sept. 11 attacks. It features 184 benches, each one etched with the name of a victim who died at the Pentagon. Each bench sits above its own reflection pool.