Among the few native states in insular Southeast Asia that were able ito resist European conquest until the end of the nineteenth century, the Sultanate of Sulu is doubly interesting because it was located strategically on an important trade route between the Celebes Sea and the South China Sea. While the Spanish destroyed the fledgling Islamic principality near Manila, their attempts to conquer the stronger Moslem societies in Mindanao and Sulu were frustrated at every turn., and were an enormous drain on the royal treasury. While they were in control of the main town of Sulu between 1635 and 1646, they were unable to hold it, and settled for a treaty with the sultan instead, withdrawing to the newly established garrison at Zamboanga. For the next two hundred and fifty years the Spanish and the Sulus, or Tausug as they call themselves, were engaged in almost continuous warfare, which ended only when the Spanish left the Philippines in 1899. This history of continuous warfare had a formative effect on religious development in the area. While Islam had probably begun to penetrate the Sulu several hundred years before the Spanish arrived, much of the process of gradual Islamization was played out against the background of this war. Faced with an ethnocentric and1 militant Christian mis sionary zeal to the north, the Tausug conception of Islam grew naturally to emphasize the militancy of the holy war, or jihad, against the non believers.