The Pan-Arab colors were first introduced in 1916, with the Flag of the Arab Revolt. Although they represent secular Arab nationalism as opposed to Islamism, the choice of colours has been explained by Islamic symbolism in retrospect, so by Mahdi Abdul Hadi in Evolution of the Arab Flag (1986): black as the Black Standard of Muhammad, the Rashidun Caliphate and the Abbasid Caliphate, white as the flag of the Umayyad Caliphate, green as the flag of the Fatimid Caliphate and red as the flag of the Khawarij. On 30 1917 Hussein bin Ali, Sharif of Mecca, leader of the Arab Revolt replaced his plain red flag with one horizontally striped in black, green, and white with a red triangular area at the hoist.