Oman's geography makes it difficult to rule. Uniting the distinct population centers created by the country's mixture of mountains and desert has been a costly enterprise. Omani Sultan Qaboos bin Said has strengthened his country and maintained a balance of internal and external interests, but his reluctance to name a successor could have serious repercussions for Oman's future stability. Oman's two main population centers, Muscat and Salalah, occupy small coastal strips separated from the desert by mountain ranges. For centuries, the al-Said family has ruled the cosmopolitan trading hub of Muscat and other coastal communities. Territorial control expanded to Zanzibar, off the Tanzanian coast, and to Gwadar, Pakistan, in the 18th century after Oman recovered from Portuguese and Persian attempts at colonization. The country's desert interior has long been home to the Ibadi Imamate that ruled from the Omani city of Nizwa.