 Following the American occupation of the northern Philippine Islands during 1899, Spanish forces in Mindanao were cut off, and they retreated to the garrisons at Zamboanga and Jolo.American forces relieved the Spanish at Zamboanga on May 18, 1899, and at Basilan seven months after.On December 8, 1899, the Americans led by Col. James S. Petit occupied the Spanish naval base of Isabela de Basilan.