They continued in existence thereafter as senior units in the field armies of the Western and Eastern Roman Empires.

The old-established Praetorian Guard was based at the Castra Praetoria in Rome, and had frequently proved disloyal, making and deposing emperors and even on one occasion in 193 putting the Imperial throne up for auction to the highest bidder (cf: Didius Julianus).

Thus Diocletian, who ruled from Nicomedia, promoted two faithful legions from the Illyricum (Legio V Iovia and VI Herculia), the area he was also descended from, to be the personal protectors of the Roman Emperors.

The two legions however continued to be counted among the senior units of the army, and after its division between East and West, they too were divided.

[3] In 398, the Jovians and Herculians of the West were part of the small body that invaded Africa and suppressed the Gildonic revolt.