It is best known for its role in the War of 1812, when it successfully defended Baltimore Harbor from an attack by the British navy from Chesapeake Bay on September 13–14, 1814.

The sight of the ensign inspired Francis Scott Key to write the poem "Defence of Fort M'Henry" that was later set to music in the song "To Anacreon in Heaven", which was later known and designated as "The Star-Spangled Banner", the national anthem of the United States.

Fort Whetstone stood on Whetstone Point in the residential and industrial area of present-day Locust Point in Baltimore, which juts into the opening of Baltimore Harbor between the basin at the present-day Inner Harbor and Northwest branch on the north side and the Middle and Ferry (now Southern) branches of the Patapsco River on the south side.

[4] In case of such an attack on this first line of defense, each point, or bastion could provide a crossfire of cannon and small arms fire.

Beginning at 6:00 a.m. on September 13, 1814, British warships under the command of Vice Admiral Alexander Cochrane continuously bombarded Fort McHenry for 25 hours.

Only one British warship, a bomb vessel, received a direct hit from the fort's return fire, which wounded one crewman.

[7] Washington lawyer Francis Scott Key went to Baltimore to negotiate the release of Dr. William Beans, a civilian prisoner of war, and there he witnessed the bombardment from a nearby truce ship.

Key saw the flag emerge intact in the dawn of September 14,[6] and he was so moved that he began to compose "Defence of Fort M'Henry" set to the tune "To Anacreon in Heaven".

It was renamed "The Star-Spangled Banner" and became the United States' national anthem, and has even been translated into various languages, targeted mostly for US immigrants who later acquired American citizenship.

During the American Civil War the area where Fort McHenry sits served as a military prison, confining both Confederate soldiers, as well as a large number of Maryland political figures who were criticizing then President Abraham Lincoln.

The court case, Ex parte Merryman, remains unresolved, and the Executive Branch continued to refuse to comply with Taney's ruling.

During World War I, an additional hundred-odd buildings were built on the land surrounding the fort in order to convert the entire facility into an enormous U.S. Army hospital for the treatment of troops returning home from the European conflict.

After undergoing restoration at the National Museum of American History, it is now on display there in a special exhibit that allows it to lie at a slight angle in dim light.

The event included a parade of tall ships, a large fireworks show, and the Navy's Blue Angels[16] As of 2015[update], restoration efforts began to preserve the original brick used in construction of the Fort, primarily through mortar replacement.