Flavius Valerius Constantius "Chlorus" (c.250 25 July 306), also called Constantius I, was a Roman emperor as one of the four original members of the Tetrarchy established by Diocletian in 293. He was a junior-ranking emperor (caesar) from 293 to 305, and senior emperor (Augustus) from 305 to 306. Constantius was also father of Constantine the Great, the first Christian emperor of Rome. The nickname Chlorus (Greek: , lit. "the Green") was first popularized by Byzantine-era historians and not used during the emperor's lifetime.Of humble origin, Constantius had a distinguished military career and rose to the top ranks of the army. Around 289 he set aside Helena, Constantine's mother, to marry a daughter of Emperor Maximian, and in 293 was added to the imperial college by Maximian's colleague, Diocletian. Assigned to rule Gaul, Constantius defeated the usurper Carausius there and his successor Allectus in Britain, and campaigned extensively along the Rhine frontier, defeating the Alamanni and Franks. When the Diocletianic Persecution was announced in 303, Constantius ordered the demolition of churches but did not actively hunt down Christians in his domain. Upon becoming senior emperor in May 305, Constantius launched a successful punitive campaign against the Picts beyond the Antonine Wall. He died suddenly at Eboracum (York) in July the following year.

After Constantius's death, the army, perhaps at his own instigation, immediately acclaimed his son Constantine as emperor. This act contributed to the collapse of the Diocletianic tetrarchy, sparking a series of civil wars which only ended when Constantine finally united the whole Roman Empire under his rule in 324. According to the Oxford Classical Dictionary, "Constantinian propaganda bedevils assessment of Constantius, yet he appears to have been an able general and a generous ruler". His descendants, the Constantinian dynasty, ruled the Empire until the death of his grandson Julian the Apostate in 363.

Diocletian (; Latin: Gaius Aurelius Valerius Diocletianus; c. 242/245 – 311/312), nicknamed Iovius, was Roman emperor from 284 to 305. Born to a family of low status in Dalmatia, originally named Diocles, Diocletian rose through the ranks of the military to become a cavalry commander of the Emperor Carus's army. After the deaths of Carus and his son Numerian on campaign in Persia, Diocletian was proclaimed emperor by the troops. The title was also claimed by Carus's surviving son, Carinus, but Diocletian defeated him in the Battle of the Margus.

Diocletian's reign stabilized the empire and ended the Crisis of the Third Century. He appointed fellow officer Maximian as Augustus, co-emperor, in 286. Diocletian reigned in the Eastern Empire, and Maximian reigned in the Western Empire. Diocletian delegated further on 1 March 293, appointing Galerius and Constantius as junior co-emperors (each with the title Caesar), under himself and Maximian respectively. Under the Tetrarchy, or "rule of four", each emperor would rule over a quarter-division of the empire. Diocletian secured the empire's borders and purged it of all threats to his power. He defeated the Sarmatians and Carpi during several campaigns between 285 and 299, the Alamanni in 288, and usurpers in Egypt between 297 and 298. Galerius, aided by Diocletian, campaigned successfully against Sassanid Persia, the empire's traditional enemy. In 299 he sacked their capital, Ctesiphon. Diocletian led the subsequent negotiations and achieved a lasting and favourable peace.

Diocletian separated and enlarged the empire's civil and military services and reorganized the empire's provincial divisions, establishing the largest and most bureaucratic government in the history of the empire. He established new administrative centres in Nicomedia, Mediolanum, Sirmium, and Trevorum, closer to the empire's frontiers than the traditional capital at Rome. Building on third-century trends towards absolutism, he styled himself an autocrat, elevating himself above the empire's masses with imposing forms of court ceremonies and architecture. Bureaucratic and military growth, constant campaigning, and construction projects increased the state's expenditures and necessitated a comprehensive tax reform. From at least 297 on, imperial taxation was standardized, made more equitable, and levied at generally higher rates.

Not all of Diocletian's plans were successful: the Edict on Maximum Prices (301), his attempt to curb inflation via price controls, was counterproductive and quickly ignored. Although effective while he ruled, Diocletian's tetrarchic system collapsed after his abdication under the competing dynastic claims of Maxentius and Constantine, sons of Maximian and Constantius respectively. The Diocletianic Persecution (303–312), the empire's last, largest, and bloodiest official persecution of Christianity, failed to eliminate Christianity in the empire. After 324, Christianity became the empire's preferred religion under Constantine. Despite these failures and challenges, Diocletian's reforms fundamentally changed the structure of Roman imperial government and helped stabilize the empire economically and militarily, enabling the empire to remain essentially intact for another 150 years despite being near the brink of collapse in Diocletian's youth. Weakened by illness, Diocletian left the imperial office on 1 May 305, becoming the first Roman emperor to abdicate the position voluntarily. He lived out his retirement in his palace on the Dalmatian coast, tending to his vegetable gardens. His palace eventually became the core of the modern-day city of Split in Croatia.