The Tokugawa political system was a centralized one in which domestic forces potentially capable of acting independently of the authority of the shogun and shogunate were minimized to the greatest extent possible. The forces of Islam invaded the Iberian Peninsula in the eighth century and were only driven out in 1492. Moreover, the Ottoman Empire that lay to Europe's east only reached its peak under Sultan Suleyman I and would continue to expand territorially until the 1680s. And while the roots of Western society lie in Greek civilization and Christianity, both of these originated outside of Western Europe. The European pluralism extended from geography to its political structures as well. The bakuhan political system provided a lengthy period of peace, which in turn furnished important basic conditions for Japan's unification and modernization. Another important change brought about by the protracted period of peace was in the mindset of the leadership class, something that plays an extremely important role in politics.