The Dacian Wars (101-102, 105-106) were two military campaigns fought between the Roman Empire and Dacia during Emperor Trajan's rule. The conflicts were triggered by the constant Dacian threat on the Danubian Roman Province of Moesia and also by the increasing need for resources of the staggering economy of the Roman Empire.

Trajan turned his attention to Dacia, an area north of Macedon and Greece and east of the Danube that had been on the Roman agenda since before the days of Caesar when they had defeated a Roman army at the Battle of Histria. In AD 85, the Dacians swarmed over the Danube and pillaged Moesia and initially defeated an army the Emperor Domitian sent against them, but the Romans were victorious in the Battle of Tapae in 88 and a truce was drawn up.

Emperor Trajan recommenced hostilities against Dacia and, following an uncertain number of battles, defeated the Dacian general Decebalus in the Second Battle of Tapae in 101. With Trajan's troops pressing towards the Dacian capital Sarmizegetusa Regia, Decebalus once more sought terms. Decebalus rebuilt his power over the following years and attacked Roman garrisons again in 105. In response Trajan again marched into Dacia, besieging the Dacian capital in the Siege of Sarmisegetusa, and razing it to the ground. With Dacia quelled, Trajan subsequently invaded the Parthian empire to the east, his conquests taking the Roman Empire to its greatest extent. Rome's borders in the east were indirectly governed through a system of client states for some time, leading to less direct campaigning than in the west in this period.

Trajan ( /ˈtreɪdʒən/; Latin: Marcus Ulpius Nerva Traianus Augustus; 18 September 53 – 9 August 117), was Roman Emperor from 98 to 117 AD. Born into a non-patrician family in the province of Hispania Baetica, Trajan rose to prominence during the reign of emperor Domitian. Serving as a legatus legionis in Hispania Tarraconensis, in 89 Trajan supported the emperor against a revolt on the Rhine led by Antonius Saturninus.

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 the Nabataean kingdom, creating the province of Arabia Petraea. His conquest of Dacia enriched the empire greatly — the new province possessed many valuable gold mines.

Dacian War(s) may refer to: