It was also partly a response to the Investiture Controversy, which was the most significant conflict between secular and religious powers in medieval Europe. The controversy began as a dispute between the Holy Roman Emperor and the Gregorian Papacy and gave rise to the political concept of Christendom as a union of all peoples and sovereigns under the direction of the pope; as both sides tried to marshal public opinion in their favour, people became personally engaged in a dramatic religious controversy. Also of great significance in launching the crusade were the string of victories by the Seljuk Turks, which saw the end of Arab rule in Jerusalem.