Saudi Arabia is not only the homeland of the Arab peoples-it is thought that the first Arabs originated on the Arabian Peninsula-but also the homeland of Islam, the world's second-largest religion. Muhammad founded Islam there, and it is the location of the two holy pilgrimage cities of Mecca and Medina. The Islamic calendar begins in 622, the year of the hegira, or Muhammad's flight from Mecca. A succession of invaders attempted to control the peninsula, but by 1517 the Ottoman Empire dominated, and in the middle of the 18th century, it was divided into separate principalities. In 1745 Muhammad ibn 'Abd al-Wahhab began calling for the purification and reform of Islam, and the Wahhabi movement swept across Arabia. By 1811, Wahhabi leaders had waged a jihad-a holy war-against other forms of Islam on the peninsula and succeeded in uniting much of it. By 1818, however, the Wahhabis had been driven out of power again by the Ottomans and their Egyptian allies.Saudi Arabia's relations with the U.S. were strained after the Sept. 11, 2001, terrorist attacks-15 of the 19 suicide bombers involved were Saudis. Despite the monarchy's close ties to the West, much of the extremely influential religious establishment has supported anti-Americanism and Islamic militancy. In Aug. 2003, following the U.S.-led war on Iraq in March and April 2003, the United States withdrew its troops stationed in Saudi Arabia. The U.S. had maintained troops in the country for the past decade, a source of great controversy in the strongly conservative Islamic country. One of the major reasons given for the Sept. 11 attacks by Saudi terrorist Osama bin Laden was the presence of U.S. troops in the home of Islam's holiest sites, Medina and Mecca. On May 12, 2003, suicide bombers killed 34, including 8 Americans, at housing compounds for Westerners in Riyadh. Al-Qaeda was suspected. Saudi Arabia's commitment to antiterrorist measures was again called into question by the U.S. and other countries. In July, the U.S. Congress bitterly criticized Saudi Arabia's alleged financing of terrorist organizations. While the Saudi government arrested a sizable number of suspected terrorists, little was done to quell Islamic militancy in the kingdom. Several attacks against Westerners took place in 2003 and 2004.