Crisis of the Third Century

The Crisis of the Third Century (235–284 AD) was a period in which the Roman Empire almost collapsed. There were invasions, civil war, plague, and the economy collapsed. The crisis is sometimes called the "Military Anarchy" or "Imperial Crisis". 
The crisis began with the assassination of Emperor Alexander Severus at the hands of his own troops in 235 AD. This started a fifty-year period in which 20–25 rivals struggled for the throne. Most were important Roman Army generals who took control over all or part of the Empire. 
By 258–260, the Empire had split into three competing states. There was the Gallic Empire, which included the Roman provinces of Gaul, Britannia and Hispania, and the Palmyrene Empire, with the eastern provinces of Syria Palaestina and Aegyptus. These became independent of the Italian Roman Empire proper, which stood between them. The crisis ended with the ascension of Diocletian in 284.
The crisis resulted in many changes to the Empire's institutions, society, economic life and, eventually, religion. It was a transition period between classical antiquity and late antiquity.