Amir al-Mu'minin
According to Sunni Muslims, the first caliph was Abu Bakr Siddique, followed by Umar ibn al-Khattāb who was the first caliph to be called "Amir al-Mu'minin" and the second of the Four Rightly Guided Caliphs. Uthman ibn Affan and Ali ibn Abi Talib also were called by the same title, while the Shi'a consider Ali to have been the first truly legitimate caliph, although they concede that Ali accepted his predecessors because he eventually sanctioned Abu-Bakr. The rulers preceding these first four did not receive this title by consensus, and it was turned into a monarchy thereafter.