area where the Pentagon would be located, in 1878
Source: Library of Congress, Atlas of fifteen miles around Washington (by G. M. Hopkins, 1878)
The Pentagon has a Washington, D.C. mailing address. The Secretary of Defense, Joint Chiefs of Staff, Army, Air Force, Navy, and Marine Corps have six Zip Codes (20301, 20318, 20310, 20330, 20350, and 20380) assigned by the US Postal Service to Washington, DC.1
Despite the mailing address, the headquarters building for the Department of Defense is located in Virginia. Mailing addresses are not required to match up with political boundaries.
The Postal Service adopted "Zoning Improvement Plan" or Zip Codes in 1963. The first three digits direct mail to a regional sorting facility, and the last two digits identify a specific delivery post office. The boundaries of Zip Codes are designed to streamline mail delivery, and are not a geographic locator based on town/city/county boundaries.2
When the Pentagon site was chosen in 1941, it occupied in part the site of the old Washington-Hoover Airport, which had closed after the opening of National Airport. Back in 1929, Washington Airport had sued a contractor building the Mount Vernon Memorial Highway. Smoot Sand & Gravel Corporation was building a retaining wall below the high-water mark, and that wall limited potential expansion of the airport runway.
Washington Airport claimed it owned that land on the Potomac River shoreline, down to the low-water mark. In 1931, the Supreme Court ruled in Smoot Sand & Gravel Corp. v. Washington Airport, Inc. that the Virginia-District of Columbia boundary was the high-water mark.
The 1632 grant from Charles II creating the new colony of Maryland gave the Calverts the entire Potomac River, "unto the further bank of the said River." Ever since, Virginia and Maryland have disputed the exact meaning of that grant and subsequent compacts intended to determine the exact location of that shoreline.
Virginia and Maryland negotiated a deal in 1877. Under the Black-Jenkins arbitration award, Virginia was given control of the shoreline down to the low-water mark. However, Federal courts ruled that the arbitration between the two states did not affect the District of Columbia, and the District controlled the shoreline to the high-water mark.3
Dredging of the Little River channel between the Long Bridge and Analostan/Mason Island produced "spoils," the sediments scraped out of the riverbed. These were dumped on the Virginia, shoreline altering the boundary between land and water but not altering the legal boundary between Virginia and the District of Columbia that was based on the 1791 shoreline. The new land created by the dredge spoils was located within the District.
When the Pentagon was constructed in 1941, more sediments were excavated from the river bottom. The site of the Pentagon was raised a minimum of eight feet, ensuring it would not flood. As a result, there is no flood wall lining the shoreline to protect the Pentagon.
Congress passed a law in 1945 to clarify that the National Airport was located within Virginia. The law defined the high-water mark, as it existed in 1945, as the boundary line north of Alexandria.
The Pentagon was built inland from the high-water mark, so it is located within Virginia. The dredge spoils used to create Columbia Island are within the District of Columbia. Near the Pentagon, Boundary Channel marks the location of the 1791 shoreline and the border between Virginia and the District of Columbia.4
the Pentagon in located in Virginia, and Boundary Channel marks the modern edge of the District of Columbia
Source: ESRI, ArcGIS Online
Alexander Island was located east of the modern Pentagon
Source: Library of Congress, Arlington County, eighteenth century Construction of the Pentagon