Great Conspiracy in the context of Roman civil war of 350–353

Count Theodosius (Latin: Theodosius comes; died 376), Flavius Theodosius or Theodosius the Elder (Latin: Theodosius major), was a senior military officer serving Valentinian I (r. 364–375) and the Western Roman Empire during Late Antiquity. Under his command the Roman army defeated numerous threats, incursions, and usurpations. Theodosius was patriarch of the imperial Theodosian dynasty (r. 379–457) and father of the emperor Theodosius the Great (r. 379–395).

Appointed comes rei militaris per Britannias (commander of mobile military forces for the Diocese of the Britains) by Valentinian, Theodosius put down the Great Conspiracy (367–368) and the usurpation of Valentinus. After restoring order in Britain he returned to continental Europe and fought against the Alemanni; as Valentinian's magister equitum (Master of Horse) he successfully invaded Alemannic territory (371 or 370). In 372 Theodosius led a successful campaign against the Sarmatians. Within the same year Firmus, a Mauritanian prince, rebelled against Roman rule with the help of African tribes like the Abanni and Caprarienses. Theodosius was sent to Africa and in two hard-fought campaigns (373–374) put down the insurrection. In 376, after the death of emperor Valentinian, he was arrested and executed, presumably as he was seen as a threat to the new western emperors Gratian and Valentinian II.

Valentinian I (Latin: Valentinianus; 321 – 17 November 375), sometimes known as Valentinian the Great, was Roman emperor from 364 to 375. He ruled the Western half of the empire, while his brother Valens ruled the East. During his reign, he fought successfully against the Alamanni, Quadi, and Sarmatians, strengthening the border fortifications and conducting campaigns across the Rhine and Danube. Also, his general Theodosius the Elder defeated a revolt in Africa and the Great Conspiracy (a coordinated assault on Roman Britain by Picts, Scoti, and Saxons). Valentinian founded the Valentinian dynasty, with his sons Gratian and Valentinian II succeeding him in the western half of the empire and his daughter Galla marrying emperor Theodosius I.

Valentinus (fl. mid-4th century, died 369) was a Roman criminal and rebel put down after Count Theodosius's arrival in Britain in AD 369.

Ammianus Marcellinus records that Valentinus was a Pannonian whose brother-in-law Maximinus was close to the emperor Valentinian I. Having committed some serious crime, he was able to have his sentence commuted to exile to Britain, where he resided at the time of the Great Conspiracy. Ammianus does not record that Valentinus took part in that barbarian rising, but does claim that he began planning a new revolt after the arrival of Theodosius. He organized fellow exiles and attempted to bribe local troops to his cause. Discovered, he was given over to Duke Dulcitius for execution, but Ammianus notes that investigation into Valentinus's plot was cursory, lest it produce more unrest in the province.

Magnus Maximus (Classical Latin: [ˈmaːgnus ˈmaːksimus]; died 28 August 388) was Roman emperor in the West from 383 to 388. He usurped the throne from emperor Gratian.

Born in Gallaecia, he served as an officer in Britain under Theodosius the Elder during the Great Conspiracy. In 383, he was proclaimed emperor in Britannia, and in Gaul the next year, while Gratian's brother Valentinian II retained Italy, Pannonia, Hispania, and Africa. In 387, Maximus's ambitions led him to invade Italy, resulting in his defeat by Theodosius I at the Battle of Poetovio in 388. In the view of some historians, his death marked the end of direct imperial presence in Northern Gaul and Britannia.

Dulcitius may refer to either of two ancient Roman officials active in the fourth century AD.

• Dulcitius (Macedonia) was governor of Macedonia during the reign of the emperor Diocletian. He is chiefly remembered for his role in a hagiographic tale of the persecution of several Christian women in Thessalonica, in 304 AD. As such, he is the eponymous villain of Dulcitius, a 10th-century drama written by Hrotsvitha of Gandersheim.

• Dulcitius (Britannia) was a military leader praised for his abilities by the soldier-historian Ammianus Marcellinus. Although it is not actually stated by Ammianus in his original text, it is often conjectured that Dulcitius was elevated to the position of Dux Britanniarum. If this is the case, he is thought to have been brought to Britain in 369 AD by Count Theodosius in the aftermath of the Great Conspiracy, in which Roman rule on the island faced simultaneous challenge from internal rebellion and external invasion. He might have been appointed Dux as a replacement for Fullofaudes, who is likely to have been killed or lost somewhere in the north of Britain. The Roman rebel Valentinus and his associates were handed over to Dulcitius for execution.