What is a soldier of “cornet” rank, dressed in an 1830s-era uniform with bearskin plumed helmet, doing in the modern, high-tech U.S. Army? Plenty, it turns out, for the cornet is a member of the 1st Troop Philadelphia City Cavalry, a National Guard outfit that may be the oldest military unit in continuous service in the country. Its members love to parade in dress uniforms, but when war breaks out, they shed their antique finery for military fatigues and chemical-warfare suits.

Long a bastion of blue-blooded Main Liners, City Troop drew its first members in 1774 from organizations like the Gloucester Fox Hunting Club, and it often served as George Washington’s bodyguard and escort. Today it still retains the whiff of an exclusive society. In the nation’s only private armory, soldiers have after-work drinks in an oakpaneled bar below a ballroom and a library. Its current roster of 70 members, all male, of whom 30 percent are legacies, draws heavily from Philadelphia’s legal and financial circles. The unit’s calendar includes debutante balls and horse shows along with its monthly drill and maneuvers.

But once you get past these trappings and the $5,000 dress uniforms, the troopers are trained tank crews, scouts, and mortarmen. 1st City Troop drives M1-A1 Abrams tanks, and its men have participated in almost every American conflict, from the Battle of Brandywine in the Revolution to the Meuse-Argonne in World War I in the Persian Gulf.

Until the late 19705, there were several such units in the United States, but 1st City Troop is the final survivor. Can the troop’s antiquated traditions flourish in the twenty-first century? Not surprisingly, today’s young professionals are leery of months-long training commitments and the possibility of being activated in a crisis. But Capt. Eric Guenther says the troop’s very real military mission keeps it from becoming an ossified gentlemen’s club: “Our people come from all different backgrounds, and we’re not there simply to put on re-enactments or just to read about activities that others have done before us. We’re not an anachronism; we’re a very active organization that thrives today."

—Marc Goodman