Nothing in the course of Trajan’s reign was of such great and far-reaching consequence as his unfortunate and erroneous idea of defending the empire by fresh conquests, and purifying morals by the revival of military ambition. From early youth he had been trained as a soldier and general; in his campaigns he had become acquainted with many lands and nations; he was equal to all the hardships of military service, and as emperor liked to share them with his soldiers; seldom mounting his horse on the march, but going on foot like his men.
Three years after his accession he began his wars of conquest, the scene of the first being Dacia on the lower Danube. As emperor he never thought of attempts on Lower Germany, although he had acted there as governor and general for ten years. The countries of the lower Danube, and after them the East, seemed to him better suited to prove to the world his capacity as a general. In Moldavia and Wallachia some immigrants of Thracian descent, amongst whom the Dacians were the most important, had leagued themselves together, some decades before, and with their combined forces had attacked Roman Thrace. At the time when Vitellius and Vespasian were disputing the throne, they had been repulsed by the troops of the latter, on their way into upper Italy, by Thrace and Mœsia, and Fonteius Agrippa, Vespasian’s general and vice-gerent, had established a number of fortified camps on the Danube as a bulwark against them.
Under Domitian the tribes belonging to the Dacian league, with Decebalus at their head, again invaded the Roman Empire. They destroyed some fortresses, repulsed the Roman troops on several occasions, and wrought fearful havoc. Domitian himself twice marched to the Danube, but his troops were defeated in most engagements. Suspicious as he was, he dared not entrust a capable man with the command of a considerable army, although immediately after the recall of Agricola from Britain he had a general who was in every respect qualified for such a struggle. The Dacians therefore not only remained unpunished, but continued their devastations, and Decebalus actually offered the Roman emperor terms of peace on condition that he should be paid a sum of money annually. Domitian agreed to these shameful terms, and the degenerate senate of Rome granted him the honours of a triumph as conqueror of the Dacians.
Trajan pretermitted the payment of tribute, and the Dacians again invaded Roman territory. He therefore betook himself to the Danube in person, in order to undertake the conduct of the war against them (101). He crossed the river, avenged the havoc wrought by the Dacians by far worse devastations in their own land, and defeated the troops of the enemy wherever they opposed him. In the third year of the war (103) the king of the barbarians was compelled to submit and accept the terms of peace dictated by Trajan.
Xiphilinus
“He spent much on war, much also on the works of peace; but the most numerous and necessary items of expenditure had for their object the repair of roads, harbours, and public buildings, while for none of these works did he ever shed blood. There was naturally such vastness in his conceptions and in his thoughts that having caused the Circus to be raised from its ruins and rendered finer and more magnificent than before, he set up an inscription stating that he had rebuilt it so that it might contain the Roman people.