While the western parts of the Roman Empire gradually succumbed to barbarian invaders, the Byzantine Empire survived Germanic, Persian, and Arab attacks (Map 7.4). In 540 the Huns and Bulgars crossed the Danube and raided as far as southern Greece. In 559 a force of Huns and Slavs reached the gates of Constantinople. In 583 the Avars, a mounted Mongol people who had swept across Russia and southeastern Europe, seized Byzantine forts along the Danube and reached the walls of Constantinople. Between 572 and 630 the Sassanid Persians posed a formidable threat, and the Greeks were repeatedly at war with them. Beginning in 632 Muslim forces pressured the Byzantine Empire.

Why didn’t one or a combination of these enemies capture Constantinople as the Ostrogoths had taken Rome? The answer lies in strong military leadership and even more in the city’s location and its excellent fortifications. Justinian’s generals were able to reconquer much of Italy and North Africa from barbarian groups, making them part of the Eastern Roman Empire. 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. Under the skillful command of General Priskos (d. 612), Byzantine armies inflicted a severe defeat on the Avars in 601, and under Emperor Heraclius I (r. 610–641) they crushed the Persians at Nineveh in Iraq. Massive triple walls, built by the emperors Constantine and Theodosius II (408–450) and kept in good repair, protected Constantinople from sea invasion. Within the walls huge cisterns provided water, and vast gardens and grazing areas supplied vegetables and meat, 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. The site was not absolutely impregnable — as the Venetians demonstrated in 1204 and the Ottoman Turks in 1453 — but it was almost so. For centuries, the Byzantine Empire served as a bulwark for the West, protecting it against invasions from the East.