Located at the extreme southeastern corner of the Arabian Peninsula, the Sultanate of Oman is a mix of cosmopolitan port cities, rugged mountains and a wide desert interior. Divided by mountains and deserts from neighboring Saudi Arabia, Yemen and the United Arab Emirates, Omanis have integrated themselves with other Indian Ocean communities for much of their history. With few water resources and population centers isolated by topography, Oman's geography has fostered three distinct cores. The cosmopolitan economic elite of Muscat, the tribal groups in Salalah and the Ibadi religious core in Nizwa. Maintaining unity of these cores is Oman's primary geographic challenge. Muscat, Oman's largest port and political and economic capital, is separated from the rest of the country by the Al Hajar Mountains. The al Qamar Mountains surround Salalah, Oman's second largest city and a regional tourist destination. Dhofar's mountains and dense vegetation helped harbor members of a Marxist-inspired rebellion against Muscat's rule for much of the 1960-70s. Oman's government revenues are dependent on oil and gas reserves that are located in the central desert region south of Nizwa. This region was once home to the Ibadi Imamate that clashed with the coastal-based Sultanate for centuries. The Musandam Peninsula, northwest of Muscat, places the critical shipping lanes of the Strait of Hormuz within Oman's territorial waters. The British granted Oman control over the peninsula during their withdrawal from the region in the 1970s. Positioned between Peninsular Sunni Arab and Shiite Iranian spheres of influence, Muscat has had to pursue a flexible foreign policy strategy to balance between the two regional powers. Oman has traditionally relied on the major naval power of the day — formerly the British, today the United States — to help maintain this balance and protect its sovereignty.