"Before the Civil War, a statue of Freedom was commissioned for the apex of the new Capitol dome. American sculptor Thomas Crawford (1814–1857), working in Rome, created the plaster model of a woman standing nearly 20 feet high, dressed in classical garb. Crawford shipped the plaster model to the United States, and that's when the trouble began.
"Crawford's version of Freedom was wearing what is called the Phrygian or liberty cap. In ancient Rome the cap was the headgear of a freed slave. Its name derives from a soft, conical-shaped hat that was worn by the residents of Phrygia, a country in Asia Minor that fell under the rule of the Roman Empire. The cap was adopted by freed slaves and thus became known as a symbol of liberty; during the eighteenth century it was rediscovered by those involved in the French Revolution.
"Crawford's proposed design of Freedom's cap sparked a big controversy. The federal buildings were under the aegis of the Army Engineering Corps, headed by Secretary of War Jefferson Davis, a staunch southerner who later became president of the Confederacy. Davis took offense at the cap worn by the statue of Freedom—a cap that suggested abolishing the South's "peculiar institution" (slavery). Davis ordered Freedom's headgear changed. The headdress on the statue today is a helmet circled by stars, on which is perched the head of an eagle and an array of feathers.
"The changes to Crawford's design were incorporated into the actual statue, cast in bronze by workers at Clark Mills's Bladensburg foundry, just outside of D.C. In another great irony of American history, the nearly eight-ton statue of Freedom--the crowning glory of the monument to the Union--was cast by twelve slaves. When it was finally hoisted into place on December 2, 1863--again, by slaves--soldiers garrisoned in forts throughout the city fired their guns in salute...."
--DeAngelis, It Happened in Washington, D.C., p. 16–17
You can see the new helmet, which replaced the cap, here.
A soft, conical cap with the top pulled forward, worn in antiquity by the inhabitants of Phrygia, a region of central Anatolia. In paintings and caricatures, the Phrygian cap represents freedom and the pursuit of liberty.