A Question of Alliances

Following his victory in Ravenna, Marcellus found himself an emperor. However, his Empire was divided, and facing many threats on its borders. Luckily for him, Constantine was fighting a war in Hispania, and word had not yet reached the region of Honorius' demise. Nor had it spread to the Eastern Roman Empire. 

Marcellus had a very finite amount of time before he faced a three prong invasion from Constantine's rebellion, the Gothic Hordes under Alaric, and the Eastern Roman Empire. If such an event were to occur, his reign would last a matter of months. 

Because of this, Marcellus was forced to choose an ally from among the three. Despite sharing a similar culture and heritage, the Eastern Roman Empire was not an option. They were ruled by Honorius' nephew, and as a member of the Theodosian Dynasty, Theodosius II would never forgive Marcellus for deposing his uncle, nor was he likely to recognize Marcellus' rule as legitimate.