The statement that best summarizes the causes and effects of the expansion of the Umayyad Caliphate is: "Because of leadership changes and military conquest, the Umayyad Caliphate expanded to Iraq and centralized Islamic power."
This statement highlights both the political dynamics (leadership changes) and the means of expansion (military conquest) that contributed to the growth and centralization of power within the Umayyad Caliphate, which aligns well with historical accounts.