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The Umayyad Caliphate (Arabic: الخلافة الأموية‎, trans. Al-Khilāfat al-ʾumawiyya) was the second of the four major Islamic caliphates established after the death of Muhammad. This caliphate was centered on the Umayyad dynasty (Arabic: الأمويون‎, al-ʾUmawiyyūn, or بنو أمية, Banū ʾUmayya, "Sons of Umayya"), hailing from Mecca. The Umayyad family had first come to power under the third caliph, Uthman ibn Affan (r. 644–656), but the Umayyad regime was founded by Muawiya ibn Abi Sufyan, long-time governor of Syria, after the end of the First Muslim Civil War in 661 CE/41 AH. Syria remained the Umayyads' main power base thereafter, and Damascus was their capital. The Umayyads continued the Muslim conquests, incorporating the Caucasus, Transoxiana, Sindh, the Maghreb and the Iberian Peninsula (Al-Andalus) into the Muslim world. At its greatest extent, the Umayyad Caliphate covered 15 million km2 (5.79 million square miles), making it the largest empire (in terms of area - not in terms of population) the world had yet seen, and the fifth largest ever to exist.
How many million square miles was the Umayyad caliphate at its largest size?
5.79