In the spring of 105 AD, Dacian King Decebalus re-ignited the conflict with Rome, attacking Roman auxiliary forts and legion work parties. With the return of Trajan from Rome, the 30th left their base at Brigetio to join the army he was assembling. Despite the Dacian's early successes against the Romans prior to Trajan's return, the full weight of the legions brought the Second Dacian War to an end. The Dacians were defeated and King Decebalus committed suicide rather than be captured.

Subsequent the victory the 30th was re-assigned to the Lower Rhine frontier at Noviomagnus where it would remain until the fourth century. In 359 AD along with six other legions the 30th was sent to Mesopotamia in defense of the city of Aida, which was facing a large Persian army. After a seventy-three day siege the Persian army of 100,000 overran the seven Roman legions and the 30th Ulpia ceased to exist.