Some member of the Japanese administration, realizing that the Muslims had remained separate the Spanish and American regimes,recognized the need to implement a different policy towards them. The Japanese Navy initially considered separating Mindanao from the rest of the Philippines and administering it separately. The Mindanao-Sulu area was strategic and necessary for the conduct of campaigns in the south, but military necessity and a shortage of knowledgeable bureaucrats resulted in direct military rule in Mindanao and Sulu, which was harsh, despite developments in Luzon. Many Muslims sided with the pro-American guerrilla units, and with the topography well suited to irregular warfare, Mindanao and Sulu, save for the coastal urban centers and key towns along major highways, were never successfully occupied.