northern areas). After a further year of Congressional and Virginia General Assembly negotiations, the island became part of Alexandria County (the northern portion later separated from the City of Alexandria and was renamed Arlington County). After Carter died in 1851, the island passed to William A. Bradley. Nonetheless, in 1854, the island again hosted an entertainment, this time given by the late John Mason's daughter Anna Maria (who had married Sydney Smith Lee in 1834) to mark a treaty opening Japan to foreign trade (Captain Lee having served under Commodore Matthew C. Perry on the voyage which resulted in the treaty). American Civil War In 1861, Alexandria County had about 10,000 residents, three-quarters of whom lived in the city portion, and about half of those in the country portion were enslaved or "free colored". Union forces occupied the island, Mason's former mansion, and the rest of the formerly retroceded area on the night of May 23-24, hours after Virginia voters ratified secession. Three federal units covered the three major trans-Potomac routes, and soon fortified the county as part of the military defenses of the nation's capital. Mason's Island soon became a U.S. Army training camp called Camp Greene. Following President Lincoln's decision to allow African Americans to join the U.S. Army, the 1st United States Colored Infantry used the island to train its soldiers. From 1864 to 1865, the camp housed as many as 1,200 formerly enslaved people, first under the authority of the U.S. Army, and later, the Freedmen's Bureau. Locals continued to call it "Mason's Island" until the memorial was built. Test site and gas light company Following the declaration of war against Spain in 1898, the island became a test site for a number of private experiments in electrical ignition of the explosives dynamite and joveite led by the chemist Charles Edward Munroe of Columbian University. Monroe's experiments, which explored the use of the explosives for mining waterways and roadways and preparing ground for rapid entrenchment, were conducted in secret and without alerting the District of Columbia Police Department, which investigated citizens' reports of Spanish spy activity and found the explosives and detonators buried on the island. From 1913 to 1931, Washington Gas Light Company owned the island, and allowed vegetation to grow unchecked on the island. National memorial In 1931, the Theodore Roosevelt Memorial Association purchased the island from the gas company with the intention of