Saudi Arabia, officially the Kingdom of Saudi Arabia (KSA), is a country on the Arabian Peninsula in Western Asia. It has a land area of about 2,150,000 km2 (830,000 sq mi), making it the fifth-largest country in Asia, the second-largest in the Arab world, and the largest in Western Asia. It is bordered by the Red Sea to the west; Jordan, Iraq, and Kuwait to the north; the Persian Gulf, Qatar and the United Arab Emirates to the east; Oman to the southeast; and Yemen to the south. Bahrain is an island country off the east coast. The Gulf of Aqaba in the northwest separates Saudi Arabia from Egypt. Saudi Arabia is the only country with a coastline along both the Red Sea and the Persian Gulf, and most of its terrain consists of arid desert, lowland, steppe, and mountains. Its capital and largest city is Riyadh. The country is home to Mecca and Medina, the two holiest cities in Islam. Pre-Islamic Arabia, the territory that constitutes modern-day Saudi Arabia, was the site of several ancient cultures and civilizations; the prehistory of Saudi Arabia shows some of the earliest traces of human activity in the world. The world's second-largest religion, Islam, emerged in what is now Saudi Arabia. In the early 7th century, the Islamic prophet Muhammad united the population of Arabian Peninsula and created a single Islamic religious polity. Following his death in 632, his followers rapidly expanded the territory under Muslim rule beyond Arabia, conquering huge and unprecedented swathes of territory (from the Iberian Peninsula in the west to parts of Central and South Asia in the east) in a matter of decades. Arab dynasties originating from modern-day Saudi Arabia founded the Rashidun (632–661), Umayyad (661–750), Abbasid (750–1517), and Fatimid (909–1171) caliphates, as well as numerous other dynasties in Asia, Africa, and Europe. The area of modern-day Saudi Arabia formerly consisted of mainly four distinct historical regions: Hejaz, Najd, and parts of Eastern Arabia (Al-Ahsa) and South Arabia ('Asir). The Kingdom of Saudi Arabia was founded in 1932 by King Abdulaziz (known as Ibn Saud in the West). He united the four regions into a single state through a series of conquests beginning in 1902 with the capture of Riyadh, the ancestral home of his family, the House of Saud. Saudi Arabia has since been an absolute monarchy, where political decisions are made on the basis of consultation among the King, the Council of Ministers, and the country’s traditional elites that oversee a highly authoritarian regime. The ultraconservative Wahhabi religious movement within Sunni Islam has been described as a "predominant feature of Saudi culture", although the power of the religious establishment has been significantly eroded in the 2010s. In its Basic Law, Saudi Arabia continues to define itself as a sovereign Arab Islamic state with Islam as its official religion, Arabic as its official language, and Riyadh as its capital. Petroleum was discovered on 3 March 1938 and followed up by several other finds in the Eastern Province. Saudi Arabia has since become the world's second-largest oil producer (behind the US) and the world's largest oil exporter, controlling the world's second-largest oil reserves and the fourth-largest gas reserves. The kingdom is categorized as a World Bank high-income economy and is the only Arab country to be part of the G20 major economies.The kingdom spends 8% of its GDP on the military (highest in the world after Oman), which places it as the world's third biggest military spender behind the United States and China, and the world's largest arms importer from 2015 to 2019, receiving half of all the US arms exports to the Middle East. According to the BICC, Saudi Arabia is the 28th most militarized country in the world and possesses the second-best military equipment qualitatively in the region, after Israel. By the late 2010s, there have been continual calls for halting of arms sales to Saudi Arabia, mainly due to alleged war crimes in Yemen and especially following the assassination of Jamal Khashoggi. The state has attracted criticism for a variety of reasons, including its role in the Yemeni Civil War, alleged sponsorship of Islamic terrorism and its poor human rights record, which has been characterized by the excessive and often extrajudicial use of capital punishment, failure to adopt adequate measures against human trafficking, state-sponsored discrimination against religious minorities and atheists, and antisemitism, and its strict interpretation of Sharia law.Saudi Arabia is considered both a regional and middle power. The Saudi economy is the largest in the Middle East; the world's eighteenth-largest economy by nominal GDP and the seventeenth-largest by PPP. As a country with a very high Human Development Index, it offers a tuition-free university education, no personal income tax, and a free universal health care system. Saudi Arabia is home to the world's third-largest immigrant population. It also has one of the world's youngest populations, with approximately 50 per cent of its population of 34.2 million being under 25 years old. In addition to being a member of the Gulf Cooperation Council, Saudi Arabia is an active and founding member of the United Nations, Organisation of Islamic Cooperation, Arab League, Arab Air Carriers Organization and OPEC.