For the first four centuries following the birth of Christ, most of Great Britain and western Europe was under the control of the Roman Empire. From time to time, the Celtic tribes who had inhabited Britain and France for several hundred years rose up in revolt. But the Roman army was so efficient that it kept even the far reaches of the empire in a state of relative peace, stability, and prosperity, sometimes called the Pax Romana, or "Roman peace."

Then, in about the third century A. D. the Roman Empire began a slow political and economic decline. So-called barbarian tribes— the Vandals, the Huns, the Visigoths, and the Franks—took advantage of the empire's weakness to invade it from the East. As the Roman armies withdrew from France and England to defend the heart of the empire, those countries, too, were...