The recognition of Mu'awiya in Kufa, referred to as the "year of unification of the community" in the Muslim traditional sources, is generally considered the start of his caliphate. With his accession, the political capital and the caliphal treasury were transferred to Damascus, the seat of Mu'awiya's power. Syria's emergence as the metropolis of the Umayyad Caliphate was the result of Mu'awiya's twenty-year entrenchment in the province, the geographic distribution of its relatively large Arab population throughout the province in contrast to their seclusion in garrison cities in other provinces, and the domination of a single tribal confederation, the Kalb-led Quda'a, as opposed to the wide array of competing tribal groups in Iraq. The long-established, formerly Christian Arab tribes in Syria, having been integrated into the military of the Byzantine Empire and their Ghassanid client kings, were "more accustomed to order and obedience" than their Iraqi counterparts, according to the historian Julius Wellhausen. Mu'awiya relied on the powerful Kalbite chief Ibn Bahdal and the Kindite nobleman Shurahbil ibn Simt alongside the Qurayshite commanders al-Dahhak ibn Qays al-Fihri and Abd al-Rahman, the son of the prominent general Khalid ibn al-Walid, to guarantee the loyalty of the key military components of Syria. Mu'awiya preoccupied his core Syrian troops in nearly annual or bi-annual land and sea raids against Byzantium, which provided them with battlefield experience and war spoils, but secured no permanent territorial gains. Toward the end of his reign the caliph entered a thirty-year truce with Byzantine emperor Constantine IV (), obliging the Umayyads to pay the Empire an annual tribute of gold, horses and slaves.