On October 8, Constantine I, who controlled the west, won a overwhelming victory at the Battle of Cibalae against Licinius, his co-emperor in the East.

Literary sources refer to Valens only as caesar (heir apparent), but numismatic evidence clearly indicates that he was augustus.

According to Petrus Patricius, Constantine was infuriated by the news of Valens' promotion:[6]"The emperor made clear the extent of his rage by his facial expression and by the contortion of his body.

Almost unable to speak, he said, "We have not come to this present state of affairs, nor have we fought and triumphed from the ocean till where we have now arrived, just so that we should refuse to have our own brother-in-law as joint ruler because of his abominable behaviour, and so that we should deny his close kinship, but accept that vile slave [i.e. Valens] with him into imperial college".

According to the Origo Constantini Imperatoris, "Valens was ordered to return again to his former private station; when that was done, peace was concluded by both emperors, with the stipulation that Licinius should hold the Orient, Asia, Thrace, Moesia, and Lesser Scythia.