Comprising a majority of the Arabian Peninsula, the Kingdom of Saudi Arabia is surrounded by Jordan, Iraq, Kuwait, Bahrain, Qatar, United Arab Emirates, Oman and Yemen. The kingdom's main geographic challenges are controlling its sparsely populated territory and maintaining shipping lanes in the Strait of Hormuz. Surrounded by vast deserts, the central Najd plateau is the core of Saudi Arabia and is home to the country's capital, Riyadh. Mecca and Medina, Islam's two holiest cities, are protected by mountains in the west and flanked by the Red Sea. The security of these cities is a key to the monarchy's legitimacy. The collapse of the Ottoman Empire after World War I set the stage for the House of Saud to unify the territory that became Saudi Arabia in 1932. Oil and natural gas have been the country's main exports and source of wealth since their discovery in 1938. A restive Eastern Province holds a majority of the kingdom's reserves and makes the country vulnerable to invasion from the north or the Persian Gulf. Cross border tribal ties also expose the kingdom to unrest in the south on the border with Yemen. A scarcity of fresh water and a small population density limit Saudi Arabia's defensive capabilities. Since the vast majority of Saudi oil exports pass via the Strait of Hormuz chokepoint, the kingdom has historically relied on a foreign power to balance regional rival, Iran and keep the Strait open. As global oil consumption continues to rise, Saudi Arabia will continue to play a dominant role in the Middle East despite its geographic challenges.