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Arch of Galerius, Thessaloniki, Greece

Gallienus and the Third Century Crisis

Third century emperors relied on the army for their position more than on any other element of Roman society. This made them especially vulnerable to usurpation by successful regional generals.

The rise of new external enemies, such as the Goths and the Sassanid Persians, exacerbated the problem by creating multiple crises on different frontiers. For significant periods, usurpers based at Trier in Germany and Palmyra in Syria hived off groups of provinces as independent 'empires'.

Gallienus (ruled 253 - 268 AD) held onto the central third of the empire after his father was defeated and captured by the Persians in 260 AD. He had the advantage of controlling the Danubian provinces, which produced the best troops of the empire, and he maximised the impact of his cavalry resources by concentrating them in a single mobile army based in northern Italy, under the command of Aureolus.

Institutionally, this cavalry force had been developing since the time of Severus, but Gallienus put a new emphasis on mobility which had implications for later emperors. Gallienus was assassinated after Aureolus rose against him.