Over the course of his reign Diocletian instituted a number of reforms to the military structure that helped transform the legions into a new kind of army.
The History of Rome - 124- The Tetrarchs at War
In the mid-to-late 290s the Imperial Tetrarchy was at war on multiple fronts. In the west Constantius undertook the reconquest of Britain, while in the east, Galerius fought a newly hostile Sassanid Empire.
The History of Rome - 123- The Tertrarchy
In 293 AD Diocletian and Maximian invited Constantius and Galerius to share in their Imperial burdens, forming what we today call the Tetrarchy.
The History of Rome - 122- Jupiter and Hercules
Immediatly after becoming the undisputed Emperor in 285, Diocletian elevated Maximian to serve as his Imperial colleague.
The History of Rome - 121- Phase Three Complete
The Crisis of the Third Century finally ended with the mini dynasty of Carus and his two sons. In 284 Diocletian rose to power, ushering in a new age in Roman history.
The History of Rome - 120- Interregnum
After Aurelian's death, an old Senator named Marcus Cluadius Tacitus briefly reigned before the throne fell to Probus, who ruled from 276-282.
The History of Rome - 119- Restitutor Orbis
Aurelian defeated the breakaway western provinces and reunified the Empire in 274 AD. The next year he was assassinated by officers who had been tricked into committing murder.
The History of Rome - 118- The Palmyrene Wars
In 272 Aurelian finally managed to bring the east back under Roman control by defeating Queen Zenobia of Palmyra.
The History of Rome - 117- Aurelian’s Walls
Aurelian became Emperor in 270 and immediatly faced an invasion of Italy by the Juthungi. After succesfully driving the Germans off, Aurelian turned his attention to building a new wall circuit around Rome to protect the capital in the future.
The History of Rome - 116- Here Come the Illyrians
Claudius Gothicus became Emperor in 268 and promptly lead the legions to victories against the Goths and the Alamanni. Unfortunately he died before he was able to reunify the Empire.