Short answers, pulled from the story.
Gordian III was thirteen years old when he became sole Roman emperor, making him the second-youngest sole emperor of the united Roman Empire. He was proclaimed emperor in approximately August 238 AD after the Praetorian Guard killed his predecessors Pupienus and Balbinus.
Timesitheus, appointed praetorian prefect in 241 and father-in-law to Gordian III, became the de facto ruler of the Roman Empire. After Timesitheus died under unclear circumstances during the eastern campaign, Marcus Julius Philippus, known as Philip the Arab, stepped in as the new Praetorian Prefect.
The exact circumstances of Gordian III's death around February 244 are disputed. He most likely died at the Battle of Misiche during the campaign against the Sassanids. A Sassanid inscription claims he died in battle near modern Fallujah, Iraq; Roman sources say he died as the army withdrew westward; and the writer Zonaras states he fell from his horse during battle.
The Battle of Resaena in 243 was a Roman victory over the Sassanids under Shapur I, fought during Gordian III's eastern campaign. The Sassanids were driven back across the Euphrates, and Gordian was planning a deeper invasion when his father-in-law and de facto administrator Timesitheus died, undermining both the campaign and the emperor's security.
Opening the doors of the Temple of Janus was the Roman ritual declaration of war. When the Sassanids under Shapur I invaded Mesopotamia, Gordian III opened those doors to signal that the empire was formally at war. Roman sources describe it as the last time the Temple of Janus was opened in Roman history.
Gordian I was Gordian III's grandfather, and Gordian II was his uncle. Both were proclaimed joint emperors during a rebellion in the Africa Province in 238, but the revolt was crushed within a month by Cappellianus, the governor of Numidia and a supporter of Maximinus Thrax.