Trajan (/?tre?d??n/; Latin: Imperator Caesar Nerva Traianus Divi Nervae filius Augustus;^[1] September 18, 53 – August 8, 117 AD) was Roman emperor from 98 AD until his death in 117 AD. Officially declared by the Senate as optimus princeps ("the best ruler"), Trajan is remembered as a successful soldier-emperor who presided over the greatest military expansion in Roman history, leading the empire to attain its maximum territorial extent by the time of his death. He is also known for his philanthropic rule, overseeing extensive public building programs and implementing social welfare policies, which earned him his enduring reputation as the second of the Five Good Emperors who presided over an era of peace and prosperity in the Mediterranean world.

Born into a non-patrician family of Italian origin in the city of Italica in the province of Hispania Baetica,^[2] Trajan rose to prominence during the reign of emperor Domitian. Serving as a legatus legionis in Hispania Tarraconensis, in 89 Trajan supported Domitian against a revolt on the Rhine led by Antonius Saturninus.^[3] In September 96, Domitian was succeeded by Marcus Cocceius Nerva, an old and childless senator who proved to be unpopular with the army. After a brief and tumultuous year in power, a revolt by members of the Praetorian Guard compelled him to adopt the more popular Trajan as his heir and successor. Nerva died on 27 January 98, and was succeeded by his adopted son without incident.

As a civilian administrator, Trajan is best known for his extensive public building program which reshaped the city of Rome and left multiple enduring landmarks such as Trajan's Forum, Trajan's Market and Trajan's Column. Early in his reign, he annexed theNabataean kingdom, creating the province of Arabia Petraea. His conquest of Dacia enriched the empire greatly — the new province possessed many valuable gold mines. However, the new province's exposed position to the north of the Danube made it susceptible to attack on three sides, and it was later abandoned by Emperor Aurelian.

His war against the Parthian Empire ended with the sack of the capital Ctesiphon and the annexation of Armenia and Mesopotamia. His campaigns expanded the Roman Empire to its greatest territorial extent. In late 117, while sailing back to Rome, Trajan fell ill and died of a stroke in the city of Selinus. He was deified by the Senate and his ashes were laid to rest under Trajan's Column. He was succeeded by his adopted son Hadrian.

Decebalus (ruled 87-106) was the last king of Dacia. He is famous for fighting three wars, with varying success, against the Roman Empire under two emperors. After raiding across the Danube, he defeated a Roman invasion in the reign of Domitian, securing a period of independence during which Decebalus consolidated his power.

When Trajan came to power, he invaded Dacia to weaken its threat to Roman border territory. Decebalus was defeated. He remained in power as a client king, but continued to assert his independence, leading to a final and overwhelming Roman invasion in 105. Trajan reduced the Dacian capital Sarmizegetusa in 106, absorbing Dacia into the Empire. Decebalus committed suicide to avoid capture.

Since the mid 19th century Decebalus has been portrayed as a national hero in Romania, the modern successor to ancient Dacia. There are several monuments depicting him.