It would be hard if not outright impossible to overstate the impact Roman Emperor Constantine I had on the history of Christianity, Ancient Rome, and Europe as a whole. Constantine remains an influential and controversial figure to this day. He achieved enduring fame by being the first Roman emperor to personally convert to Christianity, and for his notorious Edict of Milan, the imperial decree which legalized the worship of Christ and promoted religious freedom throughout the Empire. More than 1,500 years after Constantine’s death, Abdu'l-Bahá, the head of the Bahá'í Faith, wrote, “His blessed name shines out across the dawn of history like the morning star, and his rank and fame among the world's noblest and most highly civilized is still on the tongues of Christians of all denominations.”
However, it can be argued that despite his military successes - the most notable of which occurred fighting for supremacy against other Romans - Constantine may well have set the stage for the ultimate collapse of the Roman Empire as it had existed up until that point. It was Constantine who first decided that Rome, exposed and vulnerable near the gathering masses of barbarians moving into Germania and Gaul, was a strategically unsafe base for the Empire, and thus expanded the city of New Rome on the Dardanelles straits, creating what eventually became Constantinople.
In terms of geopolitics, perhaps the most seminal event of the Middle Ages was the successful Ottoman siege of Constantinople in 1453. The city had been an imperial capital as far back as the 4th century, when Constantine the Great shifted the power center of the Roman Empire there, effectively establishing two almost equally powerful halves of antiquity’s greatest empire. Constantinople would continue to serve as the capital of the Byzantine Empire even after the Western half of the Roman Empire collapsed in the late 5th century.
The end of the Byzantine Empire had a profound effect not only on the Middle East but Europe as well. Constantinople had played a crucial part in the Crusades, and the fall of the Byzantines meant that the Ottomans now shared a border with Europe. The Islamic empire was viewed as a threat by the predominantly Christian continent to their west, and it took little time for different European nations to start clashing with the powerful Turks.
The Ottoman conquest of Constantinople also played a decisive role in fostering the Renaissance in Western Europe. The Byzantine Empire’s influence had helped ensure that it was the custodian of various ancient texts, most notably from the ancient Greeks, and when Constantinople fell, Byzantine refugees flocked west to seek refuge in Europe. Those refugees brought books that helped spark an interest in antiquity that fueled the Italian Renaissance and essentially put an end to the Middle Ages altogether.