As soon as Trajan became Roman emperor, he planned a campaign against Dacia. This campaign resulted in the first Dacian war fought between 101 and 102.
The reason used for this campaign was Decebalus' lack of respect for the Romans, and the fact that he failed to respect the agreements of the peace reached following the First Battle of Tapae from 87/88.
The Roman army crossed the Danube at Viminacium, slowly making its way into Dacia. Just like in 87/88, the battle took place at Tapae. The Dacians resisted the Roman offensive, but as a storm broke out, the Dacians believing it is a sign from the gods, decided to withdraw.
Because the winter was near, Trajan decided to wait until spring to continue his offensive on Sarmizegetusa. Decebalus took advantage of the new situation, and in the winter of 101 to 102, he attacked the Roman province of Moesia, a major clash taking place at the Battle of Adamclisi.