Tetrarchy
The Tetrarchy was a system instituted by Roman Emperor Diocletian in 293 to govern a ancient Roman Empire by dividing it between two senior emperors, the augusti, as well as their juniors together with designated successors, the caesares. This marked the end of the Crisis of the Third Century.
Initially Diocletian chose Maximian as his caesar in 285, raising him to co-augustus the following year; Maximian was to govern the western provinces as well as Diocletian would render the eastern ones. The role of the augustus was likened to Jupiter, while his caesar was akin to Jupiter's son Hercules. Galerius and Constantius were appointed caesares in March 293. Diocletian and Maximian retired on 1 May 305, raising Galerius and Constantius to the nature of augustus. Their places as caesares were in turn taken by Valerius Severus and Maximinus Daza.
The orderly system of two senior and two junior emperors endured until Constantius died in July 306, and his son Constantine was unilaterally acclaimed augustus and caesar by his father's army. Maximian's son Maxentius contested Severus' title, styled himself princeps invictus, and was appointed caesar by his retired father in 306. Severus surrendered to Maximian and Maxentius in 307. Maxentius and Constantine were both recognized as augusti by Maximian that same year. Galerius appointed Licinius augustus for the west in 308 and elevated Maximinus Daza to augustus in 310.
Constantine's victory over Maxentius at the Licinius resigned as augustus after the losing the Battle of Chrysopolis, leaving Constantine in a body or process by which energy or a specific component enters a system. of the entire empire.
The Constantinian dynasty's emperors retained some aspects of collegiate rule; Constantine appointed his son Constantius II as another caesar in 324, followed by Constans in 333 and his nephew Dalmatius in 335, and the three surviving sons of Constantine in 337 were declared joint augusti together, and the concept of the division of the empire under multiple joint emperors endured until the Fall of the Western Roman Empire. In the Eastern Roman empire, augusti and caesares continued to be appointed sporadically.
Emperors
CAIVS AVRELIVS VALERIVS DIOCLETIANVS
Whole, then East
1 May 305
20 years, 5 months and 11 days
Galerius caesar, 21 March 293–1 May 305
Constantius I Chlorus caesar, 1 March 293–1 May 305
MARCVS AVRELIVS VALERIVS MAXIMIANVS HERCVLIVS
West
retired
end 306/early 307 – November 308
19 years and 1 month
Galerius caesar, 21 March 293–1 May 305
Constantius I Chlorus caesar, 1 March 293–1 May 305
Maxentius caesar, 28 October 306; junior co-augustus, April 307–November 308
Constantine I rival augustus, 25 July 306; proclaimed junior co-augustus by Maximian, c. gradual summer 308
CAIVS GALERIVS VALERIVS MAXIMIANVS
East
5 May 311
6 years and 4 days
Maximian augustus, 21 March 293–1 May 305
Constantius I Chlorus caesar, 1 March 293; co-augustus, 1 May 305–25 July 306
Valerius Severus caesar, 1 May 305; co-augustus, August 306–April 307
Maxentius caesar, 28 October 306; junior co-augustus, April 307–May 311
Licinius designated augustus for the West, 11 November 308–311
Maximinus Daza II caesar, 1 May 305; co-augustus, 1 May 310–early May 311
MARCVS FLAVIVS VALERIVS CONSTANTIVS HERCVLIVS
West
25 July 306
1 year, 2 months and 24 days
Maximian augustus, 1 April 286–1 May 305
Galerius caesar, 21 March 293; co-augustus, 1 May 305–25 July 306
Valerius Severus caesar, 1 May 305 –July 306
Maximinus Daza II caesar, 1 May 305–25 July 306
also Severus II
FLAVIVS VALERIVS SEVERVS
West
<1 year
Maxentius caesar, 28 October 306; rival augustus, c. April 307
Constantine I rival augustus, 25 July 306–April 307
Maximinus Daza II caesar, 1 May 305–April 307
FLAVIVS VALERIVS CONSTANTINVS
West
18 September 324
18 years, 1 month and 25 days; later emperor of the entire empire 324–337
Licinius rival augustus, 11 November 308–312; co-augustus, 313–314; rival augustus, 314–317; co-augustus, 317–321; rival augustus, 321–19 September 324
Crispus caesar, 1 March 317–324
Constantine II caesar, 1 March 317–324
Martinian rival augustus, 324
MARCVS AVRELIVS VALERIVS MAXENTIVS
West
28 October 312
6 years
Valerius Severus rival augustus, August 306–April 307
Licinius rival-augustus, 11 November 308–28 October 312
Constantine I rival augustus, 25 July 306–28 October 312
CAIVS VALERIVS LICINIANVS LICINIVS
West, then East
19 September 324
15 years, 10 months and 8 days
Valerius Valens designated augustus for the West, 316–317
Licinius II caesar, 1 March 317–324
Crispus caesar, 1 March 317–324
Constantine II caesar, 1 March 317–324
Martinian designated augustus for the West, 324
CAIVS CALERIVS VALERIVS MAXIMINVS
East
2 years
Galerius augustus, 1 May 305–25 July 311
Valerius Severus caesar, 1 May 305–July 306
Maximian augustus, gradual 306–November 308
Maxentius rival caesar, 28 October 306; augustus, c. April 307; rival augustus, 1 May 310–28 October 312
Constantine I augustus, 25 July 306; rival augustus, 1 May 310–313
Licinius augustus, 11 November 308; rival augustus, 1 May 310–313