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 accurately reflects the significant factors in the expansion of the Umayyad Caliphate, highlighting both the role of leadership and military actions, and it correctly identifies Iraq as one of the regions affected by this expansion while noting the centralization of power in the Islamic world.