The Roman Empire reached its peak of influence not long after iron replaced bronze as the metal of choice for weapons and tools … and thereafter collapsed to bring about what is known as the Dark Age. There are numerous reasons for the decline and fall of Rome, among them military defeats at the hands of the so-called barbarian tribes. In 378 AD, Gothic horsemen trounced the Romans at the Battle of Adrianople, which signaled the beginning of the dominance of cavalry on the battlefield. Rome itself was finally sacked in 410 AD by the Visigoths.

After Rome's collapse, pockets of civilization continued to exist, most notably in the Byzantine, or "Eastern Roman," Empire. The capital at Constantinople, formerly Byzantium, was established in 330 AD by Roman Emperor Constantine, who is also credited with legitimizing Christianity in Rome. The Eastern Roman Empire continued on for some 1,000 years after the fall of its Western counterpart, becoming a power in its own right. Byzantine warships were formidable vessels that made use of the latest rigging and armament technologies, such as the lateen sail and ballistae capable of hurling missiles hundreds of yards. On land, the core of the Byzantine army after the 8th Century was the cataphract, a heavily armed and armored cavalryman.

Leaders in Constantinople hoped, eventually, to regain full control of the West and unite the divided empire. But widening religious differences led the Roman church to resist reunification. In 800 AD, after Charlemagne, the Frankish King, had consolidated his power in Western Europe, Pope Leo III crowned him the new Roman Emperor, largely for protection. The Eastern and Western churches remain separate to this day.