Ibn Tamiyya (1263?-1328), formally Taqi al-Deen Ahmad Ibn Taymiyya, is seen as the theoretical father of modern Salafism, a radically conservative form of Islam. Part of the Hanbali school of Islam, he wrote on the necessity of armed jihad against foreign invasion of Islamic lands; he suggested that armed jihad should be added to the pillars of Islam. He also condemned Sufism. Wahhabism also draws on his thinking.
His core argument was expressed in urging Muslim fight against the Mongol invasion (1294-1303), in spite of protests that the Mongol monarch had converted to Islam. That king, however, allowed Mongol tribal law to coexist with Sharia, making him an apostate and a legitimate target of jihad. 
By requiring there to be no Muslim society without Islamic law, he set a context that Salafists used to justify rebellion against Muslim rulers that did not enforce that law, and was cited by the assassins of Anwar Sadat.
- Christopher Henzel (Spring 2005), "The Origins of al Qaeda's Ideology: Implications for US Strategy", Parameters, U.S. Army War College
- Steven Simon (September 14, 2006), Is there a Clash of Civilizations? Islam, Democracy, and U.S.-Middle East Policy, Council on Foreign Relations