In 633 AD, Rashidun Arabs invaded Iran and conquered it by 651 AD, largely converting Iranian people from their indigenous faiths of Manichaeism and Zoroastrianism to Sunni Islam. Arabic replaced Persian as the official language, while Persian remained the language of both ordinary people and of literature. Iran became a major contributor to the Islamic Golden Age, producing many influential scientists, scholars, artists, and thinkers. Establishment of the Safavid Dynasty in 1501, converted the Iranian people from Sunni Islam to Twelver Shia Islam, and made Twelver Shia Islam the official religion of Iran. Safavid conversion of Iran from Sunnism to Shiism marked one of the most important turning points in Iranian and Muslim history. Starting in 1736 under Nader Shah, Iran reached its greatest territorial extent since the Sassanid Empire, briefly possessing what was arguably the most powerful empire at the time. During the 19th century, Iran irrevocably lost swaths of its territories in the Caucasus which made part of the concept of Iran for centuries, to neighboring Imperial Russia. Popular unrest culminated in the Persian Constitutional Revolution of 1906, which established a constitutional monarchy and the country's first Majles (parliament). Following a coup d'état instigated by the U.K. and the U.S. in 1953, Iran gradually became close allies with the United States and the rest of the West, remained secular, but grew increasingly autocratic. Growing dissent against foreign influence and political repression culminated in the 1979 Revolution, which led to the establishment of an Islamic republic on 1 April 1979.