finally attempted to leave the harbor on July 3, the American forces destroyed or grounded five of the six ships. Only one Spanish vessel, the new armored cruiser , survived, but her captain hauled down her flag and scuttled her when the Americans finally caught up with her. The 1,612 Spanish sailors who were captured and sent to Seavey's Island at the Portsmouth Naval Shipyard in Kittery, Maine, where they were confined at Camp Long as prisoners of war from July 11 until mid-September. The Americans treated Spain's officers, soldiers, and sailors with great respect. Ultimately, Spanish prisoners were returned to Spain with their "honors of war" on American ships. Admiral Cervera received different treatment from the sailors taken to Portsmouth. For a time, he was held at Annapolis, Maryland, where he was received with great enthusiasm by the people of that city. US withdrawal Yellow fever had quickly spread among the American occupation force, crippling it. A group of concerned officers of the American army chose Theodore Roosevelt to draft a request to Washington that it withdraw the Army, a request that paralleled a similar one from General Shafter, who described his force as an "army of convalescents". By the time of his letter, 75% of the force in Cuba was unfit for service. On August 7, the American invasion force started to leave Cuba. The evacuation was not total. The U.S. Army kept the black Ninth U.S. Cavalry Regiment in Cuba to support the occupation. The logic was that their race and the fact that many black volunteers came from southern states would protect them from disease; this logic led to these soldiers being nicknamed "Immunes". Still, when the Ninth left, 73 of its 984 soldiers had contracted the disease. Puerto Rico On May 24, 1898, in a letter to Theodore Roosevelt, Henry Cabot Lodge wrote, "Porto Rico is not forgotten and we mean to have it". In the same month, Lt. Henry H. Whitney of the United States Fourth Artillery was sent to Puerto Rico on a reconnaissance mission, sponsored by the Army's Bureau of Military Intelligence. He provided maps and information on the Spanish military forces to the U.S. government before the invasion. The American offensive began on May 12, 1898, when a squadron of 12 U.S. ships commanded by Rear Adm. William T. Sampson of the United States Navy attacked the archipelago's capital, San Juan.