While the western parts of the Roman Empire gradually succumbed to barbarian invaders, the Byzantine Empire survived Germanic, Persian, and Arab attacks. In 540 a force of Xiongnu and Bulgars reached the gates of Constantinople. In 583 the Avars, a mounted Mongol people who had swept across Russia and the Balkans, seized Byzantine forts along the Danube and also reached the walls of Constantinople. Between 572 and 630 the Greeks were repeatedly at war with the Sassanid Persians (see “The Sassanid Empire and Conflicts with Byzantium”). Beginning in 632 Muslim forces pressured the Byzantine Empire (see Chapter 9).

Why didn’t one or a combination of these enemies capture Constantinople? The answer lies in strong military leadership and even more in the city’s location and excellent fortifications. During the long reign of the emperor Justinian (r. 527–565), Byzantine generals were able to reconquer much of Italy and North Africa. The Byzantines ruled most of Italy from 535 to 572 and the southern part of the peninsula until the eleventh century. They ruled North Africa until it was conquered by Muslim forces in the late seventh century. Massive triple walls, built by the emperors Constantine and Theodosius II (408–450) and kept in good repair by later emperors, protected Constantinople from sea invasion. Within the walls huge cisterns provided water, and vast gardens and grazing areas supplied food so the defending people could hold out far longer than the besieging army. Attacking Constantinople by land posed greater geographical and logistical problems than a seventh- or eighth-century government could solve. Because the city survived, the empire, though reduced in territory, endured.