I nominate Ceasar. He turned the Roman Republic into a dictatorship. The Republic might even have lasted long enough to crush , you guessed it, Mohammad!
The Roman Empire (Specifically the Eastern Roman Empire, called incorrectly the Byzantine Empire) held it's own against the Muslims, particularly the Turks, until the middle 1000's. It was the loss at the Battle of Manzikert in 1071, and more importantly, the loss of heavily populated central Anatola, that sealed the Empire's fate. Even then the Roman Empire held on until the fall of Constantinople in 1453.