Trajan's wars were, in fact, far from the only campaigns in Dacia. Trajan may actually have inherited the idea from the emperor Domitian, who fought against the Dacians between 85/6 and 88/9 into 92. He may have learned other lessons from this campaign as well – one of them being to actively publicize the campaign back in Rome – something Domitian seems to have failed to do.
In AD 85 the Dacian king, Duras, invaded Moesia, crossing the Danube and ravaging the Roman province. Moesia's governor fell fighting them in the winter of AD 85 or early 86. The commander of Duras' armies was one Diurpaneus (or Dorpaneus). He, it has been argued, was one and the same man who fought Trajan twenty years later: Decebalus.
Domitian's campaigns
Our best summary of the war comes in the epitome of Cassius Dio Book 67 (67.6.17.2). Dio calls the war serious and names the Dacian king as Decebalus (possibly a title), calling him a master of war who “showed himself as a worthy antagonist of the Romans for a long time.” Dio also accuses Domitian of not being at the front (67.6.3) but of “indulging in riotous living” in Moesia. It is entirely possible that Domitian was in fact at the front
