Gaius Messius Quintus Trajanus Decius (c. 201 – June 251), sometimes translated as Trajan Decius, was Roman emperor from 249 to 251.
Decius was one of the first among a long succession of Roman emperors (Illyrian emperors) to originate from the Danube provinces, often referred to as Illyricum. Unlike some of his immediate imperial predecessors such as Philip the Arab or Maximinus Thrax who did not have extensive administrative experience before assuming the throne, Decius was a distinguished senator who had served as suffect consul in 232, had been governor of Moesia and Germania Inferior soon afterwards, served as governor of Hispania Tarraconensis between 235 and 238, and was urban prefect of Rome during the early reign of Emperor Philip the Arab.
Around 245, Philip entrusted Decius with an important command on the Danube. By the end of 248 or 249, Decius was sent to quell the revolt of Pacatian and his troops in Moesia and Pannonia; some modern historians see this rebellion as a reflection of emerging Balkan separatism. After the collapse of the revolt, Decius let the troops proclaim him emperor. Philip advanced against him and was killed at Verona, Italy, in September 249. The Senate then recognized Decius as emperor, giving him the attribute Traianus in reference to Emperor Trajan. According to the Byzantine historian Zosimus, Decius was clothed in purple and forced to undertake the burdens of government, despite his reluctance and unwillingness.
A distinguished politician during the reign of Philip the Arab, Decius was proclaimed emperor by his troops after putting down a rebellion in Moesia. In 249, he defeated and killed Philip near Verona and was recognized as emperor by the Senate afterwards. During his reign, he attempted to strengthen the Roman state and its religion, leading to the Decian persecution, where a number of prominent Christians (including Pope Fabian) were put to death. In the last year of his reign, Decius co-ruled with his son Herennius Etruscus, until they were both killed by the Goths in the Battle of Abritus.
Decius' political program was focused on the restoration of the strength of the State, both militarily opposing the external threats, and restoring the public piety with a program of renovation of the state religion.
Either as a concession to the Senate, or perhaps with the idea of improving public morality, Decius endeavoured to revive the separate office and authority of the censor. The choice was left to the Senate, who unanimously selected Valerian (the future emperor). But Valerian, well aware of the dangers and difficulties attached to the office at such a time, declined the responsibility. The invasion of the Goths and Decius' death put an end to the abortive attempt.
During his reign, he proceeded with several building projects in Rome, including the Thermae Decianae (Baths of Decius in the Aventine), which were completed in 252 and survived through to the 16th century; Decius also repaired the Colosseum, which had been damaged by lightning strikes.