Evidence of the existence of empires dates back to the dawn of written history in Egypt and in Mesopotamia, where local rulers extended their realms by conquering other states and holding them, when possible, in a state of subjection or semisubjection. An early, highly organized empire was that of Assyria, which was succeeded by the even more integrated Persian Empire. Ancient imperialism reached its climax under the long-enduring Roman Empire, the eastern part of which lasted until late into the Middle Ages as the Byzantine Empire. In Western Europe no true empire arose to replace Rome; the Holy Roman Empire, despite the aspirations of its rulers, was little more than a confederation of princely states. However, imperialism remained an important historical force elsewhere. In the Middle East and North Africa the Arabs and later the Turks built large empires. Farther east, besides the huge, if unstable, empires of the nomadic Mongols and others arising out of Central Asia, there were long-lasting and complex imperial organizations exemplified by various Chinese dynasties.