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 Syria and centralized Islamic power.
This option accurately reflects the significant leadership changes, including the shift from the Rashidun to the Umayyad Caliphate, and emphasizes their military conquests, which were pivotal in expanding the caliphate's territory and consolidating Islamic authority.