On August 22, 392, Arbogast had Eugenius elected Western Roman Emperor. An excerpt from the article:
"The story of Eugenius is in many ways the story of the magister militum Arbogast. Eugenius was proclaimed emperor by Arbogast on 22 August 392.[[1]] This occurred three months after the death of Valentinian II, who was either murdered by Arbogastes or committed suicide in despair of the power Arbogastes held over him. Arbogastes succeeded Bauto as Valentinian II's magister militum in 388, having been one of the generals that Theodosius sent against the usurper Magnus Maximus.[[2]] Once installed, Arbogast mounted a successful punitive expedition against the Frankish leaders Marcomer and Sunno, who had ravaged Gaul during the civil war with Maximus.[[3]] It also seems that Arbogastes was the de facto ruler of the west, as the sources are unanimous that Arbogastes dominated Valentinian II and his court.[[4]] Valentinian, who was only twenty, was so distressed about this arrangement that he frequently complained to Theodosius about it.[[5]] There is reason, however, to believe that the role of Arbogast was sanctioned by Theodosius, who had reached a similar accord with Magnus Maximus in 384. In addition, after defeating Maximus in 388, Theodosius filled the highest civil offices in the west with his supporters before he returned to Constantinople in 391, so the appointment of Arbogast as "protector" of Valentinian II would have been a logical extension of this principle.[[6]]
As mentioned previously, Valentinian II died on 15 May 392, either by murder or suicide. The sources give conflicting accounts, but it most likely was a suicide. The charge of murder against Arbogast was probably propaganda after the fact to justify further Theodosius' destruction of Eugenius' regime. Theodosius' decision not to move until Eugenius was proclaimed as emperor three months later indicates that he was not overly upset with letting Arbogast run affairs in the west. If Arbogastes had murdered Valentinian II, it seems more likely that Theodosius would have seen this as an overt act of usurpation and reacted much sooner.[[7]]
Arbogast probably had hopes that he would be promoted to Augustus in the west upon the death of Valentinian II. After three months, it must have become apparent that Theodosius had no intentions of doing so, and Arbogast thus decided to revolt. He took a different approach from Maximus, however, by putting forth Eugenius as the actual imperial candidate. The reasons for this would have been two-fold. First, Eugenius would probably be a more suitable candidate to Theodosius than Arbogast himself, who was a Frank. Secondly, with Eugenius as the titular emperor, Arbogast hoped to gain support from the senatorial aristocracy of Rome. Both of these considerations were due to Eugenius' background. He was a former teacher of grammar and rhetoric, and had been magister scrinorum sometime before 392, and had become Arbogast confidant after being introduced to him by Arbogast's uncle Richomer."