Through a certain chain of events, Western Rome won against Eastern Rome (the Byzantines) and went on to reform and strengthen the empire proper. Due to this, the Romans were able to beat back the Goths and the Turks, thereby securing their power indefinitely. The empire continued on, growing farther into Europe and taking over much of Russia and west Asia. Any nations that had not been taken over properly, were allied with and absorbed into Rome as another of its smaller components. Finally, the Romans crossed the Atlantic Ocean and took over the Americas, laughing at the pathetic Native Americans who tried to fight them with stone axes and arrows.
Once the Americas were firmly within its grasp, Rome declared itself the one true nation, and the world was united under its rule. Technology thrived, and within several centuries of the conquest, humanity had surpassed even its most unlikely fantasies, and began traveling through space and inhabiting other worlds. In the end (and this is what we would consider, perhaps, the year 2000) they held full control over the known universe, and reigned supreme for all eternity. Which was fine, because everyone was happy anyway.