As part of his reorganization of the empire in 300 AD, the Emperor Diocletian separated the administration of Crete from Cyrenaica and in the latter formed the new provinces of " Upper Libya " and " Lower Libya ", using the term Libya for the first time as an administrative designation.

In 286 Diocletian had the northern provinces reorganized ; civil and military administration were separated and settlements rebuilt.

The harbour was enlarged several times under Diocletian and Constantius.

Diocletian secured the Empire's borders and purged it of all threats to his power.

Galerius, aided by Diocletian, campaigned successfully against Sassanid Persia, the Empire's traditional enemy.

The Late Roman army is the term used to denote the military forces of the Roman Empire from the accession of Emperor Diocletian in 284 until the Empire's definitive division into Eastern and Western halves in 395.

After the Crisis of the Third Century almost resulted in the Roman Empire's political collapse, the Emperor Diocletian replaced the one-headed Principate with the tetrarchy ( c. 300 AD, two Augusti ranking above two Caesares ), in which the remaining pretense of the old Republican forms was largely abandoned.

Moving the notion of the Emperor away from the republican forms of the Empire's first three centuries, Diocletian introduced a novel system of joint rule by four monarchs, the Tetrarchy.

The Dominate also featured a shift in the Empire's " center of gravity " from the west to the east, particularly after the establishment of Constantinople ; neither Diocletian nor his co-Emperor Maximian spent much time in Rome after 286, establishing their Imperial capitals at Nicomedia and Mediolanum ( modern Milan ), respectively.

The rationalis was the Roman Empire's chief financial minister prior to the reforms of Emperor Diocletian and the Late Empire.

It has even been suggested that Maximian usurped the title, and was only later recognized by Diocletian in hopes of avoiding civil war.

The historian Warren Treadgold estimates that under Diocletian the number of men in the civil service doubled from 15, 000 to 30, 000.

Without the guiding hand of Diocletian, the Empire fell into civil wars.

These developments overturned the strict division of civil and military offices, which had been one of the cornerstones of the reforms of Diocletian ( r. 284 – 305 ).

Thus the division set up by Diocletian between civil governors ( praesides etc.

In the early Byzantine period ( 4th to early 7th century ) the system of government followed the model established in late Roman times under Diocletian and Constantine the Great, with a strict separation between civil and military offices and a scale of titles corresponding to office, where membership or not in the Senate was the major distinguishing characteristic.

* Roman civil war of 284 – 285 AD, between Diocletian and Carinus-victory to Diocletian.

Diocletian tried to solve this problem by re-establishing an adoptive succession with a senior ( Augustus ) and junior ( Caesar ) emperor in each half of the Empire, but this system of Tetrarchy broke down within one generation ; the hereditary principle was re-established with generally unfortunate results, and civil war was thereafter the main method of establishing new imperial regimes.

The Crisis of the Third Century was a prolongued series of civil wars, barbarian invasions, usurpation, and ( attempted ) secession that plagued the Roman Empire from the assassination of Emperor Alexander Severus ( 235 ) to the rise to power of Diocletian ( 284 ).

The Late Roman administrative system, as established by Diocletian, provided for a clear distinction between civil and military offices, primarily to lessen the possibility of rebellion by over-powerful provincial governors.

Although Emperor Diocletian is held to have strengthened the navy, and increased its manpower from 46, 000 to 64, 000 men, the old standing fleets had all but vanished, and in the civil wars that ended the Tetrarchy, the opposing sides had to mobilize the resources and commandeered the ships of the Eastern Mediterranean port cities.

Born to a family of low status in the Roman province of Dalmatia, Diocletian rose through the ranks of the military to become cavalry commander to the Emperor Carus.

Religious legitimization elevated Diocletian and Maximian above potential rivals in a way military power and dynastic claims could not.

Diocletian argued that forbidding Christians from the bureaucracy and military would be sufficient to appease the gods, but Galerius pushed for extermination.

Aside from the rank and file legionary ( who received the base wage of 10 asses a day or 225 denarii a year ), the following list describes the system of officers which developed within the legions from the Marian reforms ( 104 BC ) until the military reforms of Diocletian ( c. 290 ).

In 293, feeling more focus was needed on both civic and military problems, Diocletian, with Maximian's consent, expanded the imperial college by appointing two Caesars ( one responsible to each Augustus ) — Galerius and Constantius Chlorus.

* Diocletian divides the empire in two, after economic and military problems.

* Emperor Diocletian conducts a military campaign in Raetia ( Switzerland ).

In 286, Diocletian elevated a military colleague, Maximian, to the throne as co-emperor of the western provinces, while Diocletian took over the eastern provinces, beginning the process that would eventually see the division of the Roman Empire into two halves, a Western and an Eastern portion.

He shared the latter title with his co-emperor and superior, Diocletian, whose political brain complemented Maximian's military brawn.

His ambitions were purely military ; he left politics to Diocletian.

Some sources ( Lactantius, Epitome ) state that Galerius hated Maxentius and used his influence on Diocletian that Maxentius be ignored in the succession ; maybe Diocletianus also thought that he was not qualified for the military duties of the imperial office.

The first is that the Greek and Latin legends of Saint Christopher identify him as belonging to the Third Valerian Cohort of the Marmantae ( Latin: Cohors tertia Valeria, at Marmantarum ), a military unit of Northern Africa of Marmarica ( between modern day Libya and Egypt ), recruited by none other than the Emperor Diocletian.

Diocletian often worked with the legion during the period of military anarchy from 235 to 284.

During the 3rd century AD, power shifted from the Italian aristocracy to a class of knights who had earned their membership by distinguished military service, often rising from the ranks: career military officers from the provinces ( especially the Balkan provinces ) who displaced the Italian aristocrats in the top military posts, and under Diocletian ( ruled 284-305 ) from the top civilian positions also.

Diocletian greatly reduced the power of these prefects as part of his sweeping reform of the empire's administrative and military structures.

The tetrarchy reform of Diocletian ( c. 296 ) multiplied the office, there was a praetorian prefect as chief of staff ( military and administrative )— rather than commander of the guard — for each of two Augusti, but not for the two Caesars.

The empire was reunited under Aurelian in 274 and reorganized by Diocletian ( from 284 ) and his successors with more emphasis on the military.

Similarly, in an edict of the Emperor Diocletian from AD 303, which set maximum prices of goods and services, the price of saddles, halters, and bridles are enumerated, as well as the price of a veterinarian for " cutting the hair and hoof of each animal.

In that year the co-emperors of the late Roman Empire resolved once again on an attempt to eradicate Christianity, the senior emperor, Diocletian, relying this time on the services of a subordinate emperor, Maximian.

Diocletian issued an Edict on Maximum Prices in 301, which attempted to establish the legal maximum prices that could be charged for goods and services.