The Pentagon is the headquarters of the United States Department of Defense, located in Arlington County, Virginia. As a symbol of the U.S. military, the "Pentagon" is often used metonymically to refer to the U.S. Department of Defense.
The Pentagon is where Magneto was imprisoned for the murder of the President sometime before 1973 when he was freed by Wolverine, Professor X, Beast and Quicksilver to help find Mystique. Magneto returned to the Pentagon to retrieve his helmet, which was kept in a vault along with Havok's damaged containment unit, the coin used to kill Sebastian Shaw and one of Angel's wings.
- For the scene where Magneto returns to the Pentagon to retrieve his helmet, the caption reads "Washington, D.C.". The Pentagon is actually located in Arlington County, across from Washington.
-  on Wikipedia