The popular interpretation of the Roman Empire falling around the middle of the Fifth Century is pretty wrong in a pretty big way. At that point, the Empire was divided into two major administrative regions. One was based in Ravenna (not even Rome!) and was down for the count by 476.
The other, based in Constantinople (not Istanbul) stuck around for another thousand years. It's known as the Byzantine Empire, and one of the reasons for its longevity was a little substance called Greek Fire, which was basically -- and maybe literally -- napalm.
Greek Fire was invented just in time to be used spectacularly in the First Arab Siege of Constantinople (not Istanbul). The city was surrounded, and the Umayyad Caliphate was feeling pretty confident that they were going to add a new member to their conquest club. For years, skirmishes were almost a daily occurrence. The Arab fleet sat in the Bosporus defying the Byzantines to come see what would happen if they tried to break out.
The Byzantine fleet started behaving strangely. In a time when naval warfare was all about smashing ships into each other as hard as you could, they were appearing in the harbor with strange siphons attached. When the two fleets finally met in a pitched battle in late 677 or early 678, the purpose of the siphons became devastatingly clear.
The Arabs got very literally hosed. With fire. Liquid fire. Liquid fire that burned with the fury of Achilles even on the surface of the ocean. The whole experience really put a damper on Umayyads' trip to Constantinople (not Istanbul), and they promptly left to write an unfavorable Yelp review.
|"There were no mints on the pillow. In fact, there were no pillows. And they set fire to my person."|
Forty years later, the Umayyad Caliphate returned to prove they hadn't learned their lesson. In what historians call one of the most important and decisive battles in history...the same thing happened again. The Second Arab Siege of Constantinople was routed by Greek Fire.
Greek Fire continued to give a ludicrously unfair advantage to the Byzantine navy until around the 13th Century. At that point, they may have lost access to the areas where ingredients for the substance could be found. We're not sure, because it was such a closely guarded secret that we don't really know what the hell Greek Fire was. The general consensus among scholars is that it was probably made with a form of petroleum and was sort of like napalm.
What we do know is that Greek Fire was so powerful that it is credited with stopping the Arab conquest of Eastern Europe dead in its tracks.