Final answer:
The subject appears to be Pupienus, a co-emperor with Balbinus, but the reference information details different historical events related to Constantine and the end of the Tetrarchy. There is a historical mismatch in the student's question which does not correspond with the provided material focusing on Constantine’s rise to power and Diocletian's reforms.
Step-by-step explanation:
The individual referred to in the student's question seems to be Pupienus, who was co-emperor with Balbinus. However, the historical account provided in the reference materials does not detail Pupienus but focuses instead on figures such as Constantine, Diocletian, and Galerius during various imperial transitions and reforms, such as the Tetrarchy and the eventual establishment of Constantine as the sole emperor. The initial description provided by the student appears to be a mix-up of different historical events and characters, as the emperors who co-reigned after the death of Gordian I and Gordian II were Pupienus and Balbinus, and they faced a different political situation than the chaos during and following the period of the Tetrarchy and the rise of Constantine.
Indeed, the assassination of co-emperor Pupienus by his Praetorian Guard while at war with Maximinus Thrax fits the student's description but does not align with the historical figures and events described in the reference material provided. Meanwhile, Diocletian and Maximian abdicated in 305 CE, leading to a complicated power struggle involving Constantine, who, after his father's death, claimed the throne and eventually defeated Maxentius at the Battle of the Milvian Bridge in 312 CE.