History of Islam
After Muhammad died, the Arab Muslim leadership was assumed by Abu Bakr. He was followed by Umar, Uthman ibn Affan, Ali, and Mu'awiya I, who established the Umayyad Caliphate. Under these caliphs, Islam became one of the most important religions in the world.  Under Umar, more than two thirds of the Eastern Roman Empire was conquered by the Arab Muslims. Umar also started the Muslim calendar. The Arab Islamic empire reached its largest geographical extent under the Umayyad Caliphate.