Today, we think of the days of empires having come and gone. Rome has been dead for 1500 years, Byzantium for 600. But, what if these empires had never fallen? The world would have been a very different place. This is where our point of divergence begins. We haven't had a large imperial power since World War I, with the defeat of Hapsburg Austria and the German Empire, and the Communization of Russia.
Point of Divergence
In 285, Emperor Diocletian of Rome orders the rapidly declining empire to be divided amongst four rulers, two in the East, two in the West. This does not fare well, and when the halves meet in battle in AD 312, Maxentius of the East is killed by Constantine of the West. Constantine reunites Rome for a short time, also making Christianity widespread in the process.
Our point diverges in AD 330, after Constantine's death, although not noticeably so until later. Emperors in this timeline in the Roman Empire (western, that is) decide to focus on militarization and make a point of ending the circuses, much to the dismay of the populace. They focus on economy and military instead. The people are unsure of this at first, but approve of the much needed tax reductions.As such, Rome is still strong in AD 476, when in our timeline, it died.