The Byzantine Empire was one of the most important nations of the Middle Ages. It was the last remnant of the Roman Empire.  Its capital, Constantinople, was probably the biggest city in the western world.  The Byzantine Empire covered most of what is modern Greece and western Turkey. Constantinople was renamed Istanbul by the Ottoman Turks, and remained the capital until Mustafa Kemal Ataturk moved it to Ankara. The Byzantines preserved Greek and Roman culture and learning–thay never had a Dark Ages.  They also defended Europe from the Arab-Islamic expansion that occurred  between the 600s and the First Crusade. The Byzantine defenses depended on two things: the walls of Constantinople, which were never breached until the Ottoman conquest in 1453; and the Byzantine navy. The coolest thing abowt the Byzantine navy was Greek fire. Greek fire was a weapon of unknown composition (but it’s thought that naturally-occurring crude oil was an ingredient). It took the form of a liquid, stored in a brass tank on a ship, which was pumped out a nozzle and set on fire, like a flamethrower. It was devastating to the wooden ships used at the time (including their own–they set themselves on fire too). Some people think that magnesium was used–magnesium burns underwater, so even jumping overboard wouldn’t save you if you were hit by it!