The Muslim conquests brought about the collapse of the Sassanid Empire and a great territorial loss for the Byzantine Empire. The reasons for the Muslim success are hard to reconstruct in hindsight, primarily because only fragmentary sources from the period have survived. Fred McGraw Donner suggests that formation of a state in the Arabian peninsula and ideological (i.e., religious) coherence and mobilization was a primary reason why the Muslim armies in the space of a hundred years were able to establish the largest pre-modern empire until that time. The estimates for the size of the Islamic Caliphate suggest it was more than thirteen million square kilometers (five million square miles).