Mount Vernon's location on the Potomac brought both opportunity and danger in wartime. During the American Revolution, in 1781 the captain of the British warship HMS Savage threatened to burn Mount Vernon unless given provisions, and Lund Washington, George Washington's cousin and farm manager, ultimately complied to protect the Mansion, a decision George Washington later condemned in a letter from April 1781. During the transfer of supplies, 17 enslaved workers from Mount Vernon boarded the Savage seeking freedom with the British, and a few months later, after the British surrender at Yorktown, 7 were returned to the estate. Before the War of 1812, George Washington had urged Secretary of the Navy Benjamin Stoddert in 1798 to fortify the Maryland shore of the Potomac to protect the new national capital, and the fort he envisioned was built in 1809 as Fort Warburton on land associated with William Digges. On August 27, 1814, days after the British burned Washington, D.C., a British naval force passed Mount Vernon on its way up the Potomac to destroy the fort, whose undermanned garrison blew it up rather than let it be captured, but Mount Vernon itself was spared by British Captain James Gordon out of respect for George Washington. During the Civil War, Mount Vernon, though in a Confederate state and only 16 miles from Washington, D.C., was treated as neutral ground: by May 1861 soldiers could visit only if they removed or covered their uniforms and laid down their arms, and in July 1861 Union General Winfield Scott ordered U.S. troops to approach the estate with reverence and leave its tomb, house, groves, and walks undisturbed. Travel there by boat became harder after April 1861 when the federal government seized the riverboat Thomas Collyer for troop transport, depriving the Mount Vernon Ladies' Association of a key means of bringing paying visitors just as it began restoring the estate. In the spring of 1862, Union Major General George B. McClellan moved his army down the Potomac toward the Chesapeake Bay and then up the James and York rivers in the campaign against Richmond, and Anne Frobel recalled the river as so crowded with vessels that it appeared almost a solid mass of white canvas.