3rd Century (Vastator Orbis)

The third century was the period of time between 201 and 300 AD. This century saw extensive turmoil within the Roman Empire, which ultimately resulted in its permanent fragmentation. This period of upheaval is known as the Crisis of the third century. Sparked by the extinction of the Severan Dynasty, the Crisis left Rome at a breaking point. With pressure on all frontiers, frequent civil wars, economic depression, and deadly plagues and famines, Rome could no longer sustain itself. In 260, Postumus led the Gallic Empire to be a de facto independent state, followed shortly by the Palmyrene Empire in 270. These states would become the first of many such 'usurper states,' whose long-standing claims for the throne resulted in the creation of practically autonomous states.

The point of divergence from our universe takes place in 270 AD. For events that took place prior to the PoD, as well as a list of what took place in our timeline, please refer to Wikipedia's article on the subject.

270 AD

 * September: Rome's Danube and Rhine borders are both under attack. Insurrections erupt throughout the border territories. Though many are simple ethnic migrations, others follow a loose confederation led by the Alemanni king Chroccus. His forces steamrolled the Upper Germanic Limes and crossed into Italy and Gaul. According to Gregory of Tours (OTL 538 - 594 AD), Chroccus' quarrel was not simply with the Romans. He intended to displace the Gallic and Germanic populations already settled in Gaul.