After the collapse of the classical Greek and Roman empires, Western European civilization struggled with each other to become the dominant empire in the region. This period of instability in Western Europe made room for a new group to dominate; the Islamic Empires. Recognized as having one of the most rapid and efficient expansions in history, the Islamic World quickly rose to create the most powerful empires during the early medieval period. Christianity quickly became the foundation used to stabilize Western Europe's warring kingdoms in the era following the collapse of Rome. The limitation of free expression, thinking, and the disallowance of outer beliefs and cultures within the religion stagnated the opportunity to expand the empire and to rebuild their economic and political structures. .
In stark contrast, their Islamic counterparts were surging ahead both geographically and economically. The appeal of Islam, due to its cultural and religious flexibility allowed Islam to establish themselves as one of the most dominant empires in history. Religion was used as a way to unite people, encourage trade, and expand the Islamic empire during this period, meanwhile Christianity withheld the West from expanding. Ultimately, the tolerance and acceptance of different ideologies and cultures and the encouragement of free thought created a sense of unity that was much more appealing compared to the restraints of the Roman Catholic Church in Western Europe which pushed outsiders to conform, and free thought to be repressed for Church theology, thus allowing the Islamic world to develop into the dominant group of the time. .
To compare the differences in how Western Europe and the Islamic World sought to expand their empires, first the role of religion in the expansion of each region's empires must be understood. Roman Catholicism was the foundation for European society; it was the one thing that united a vastly different people fragmented into small kingdoms.