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. Naturally, the Ottoman Empire would also use Constantinople as the capital of its empire after their conquest effectively ended the Byzantine Empire.In the wake of taking Constantinople, the Ottoman Empire would spend the next few centuries expanding its size, power, and influence, bumping up against Eastern Europe and becoming one of the world's most important geopolitical players. It was a rise that would not truly start to wane until the 19th century. In fact, its decline would be brought about not just because of military setbacks and government corruption, but also because of nationalist uprisings among its multiethnic territories. The fact that the other existing multicultural empire, the Austro-Hungarian Empire, also did not survive World War I makes clear just how hard governing such a vast expanse of land can be. The Ottoman Empire's inability to create a shared identity, a weak central state, and growing inner dissensions were some of the main factors explaining its long demise.Of course, before that demise, the Ottomans brought plenty of their own cultural influence to the lands they reached, and nowhere is that more evident than in the spread of Islam. While Ottoman conflicts with European powers were often portrayed as religious conflicts, the fact is that the Ottomans managed to bring Islam with them into parts of Europe, especially the Balkans, and Islam took hold in various regions, adding even more demographic complexity to a place that already had no shortage of it.