Peter Heather,The Fall of the Roman Empire: Chapter 4, "War on the Danube"

The Huns disrupted the 4th century order of barbarian kingdoms in southeastern Europe. Their attacks seem to have persuaded many of the Goths to try their luck at gaining entrance to the Roman empire. The Huns' key weapon was the very long, asymmetric, composite bow. The bow's asymmetry was the real innovation -- it allowed a longer bow, and thus one with more range and power, to be used from horseback.

Goths -- Tervingi and Greuthungi -- arrived on the Danube en masse in 376, and crossed into the empire. Contemporary sources make it out that this was welcomed, at least initially. Heather decides that this calls for extensive source criticism. In the first place, he notes, contemporaries' accounts tended to reflect imperial ideology, which could not accept the idea of barbarians having the advantage over Romans. But in this case there are reasons to think that the Romans were unsettled. The emperor Valens had committed most of his forces to an Eastern campaign, so the Romans did not have the military superiority at the frontier which had been a prerequisite for previous voluntary receptions of barbarians into the empire. The decision to admit only the Tervingi (the Greuthingi crossed the Danube later when the defenses were stripped) and the apparently rapid appeal for military assistance from the Western emperor shows there was a high level of concern. The precautionary securing of food supplies in fortified cities similarly demonstrates that the emperor thought things might go wrong. The apparent terms agreed for the settlement of the Tervingi (which allowed them to settle only in Thrace rather than in a more scattered way) shows that the situation had given them an unusual amount of leverage.

In any case, neither the Tervingi nor the Romans trusted the other side, and both apparently double-crossed the other -- the Goths by joining up with the Greuthungi, the Romans by attempting an assassination of the barbarian leadership. Things turned out badly for the Romans first with a scratch force at Marcianople in 377 and then with a full imperial army at Hadrianople in 378, where Valens attacked the Goths before additional forces from the Western emperor Gratian could arrive. Heather devotes a few pages to explaining the pressures behind Valens impetuousness. The apparent sophistication of the barbarians appreciation of the strengths and weaknesses of the Roman army seems just as notable, however. They understood that the summer heat would take a greater toll on the heavily armored Romans, and maximized their advantage by setting large wildfires too boot.

The Romans proved unable to subdue the Goths in the field, but wore them into a peace by attrition. Still, the terms left large intact settlements of Goths within the boundary of the empire -- an unfavorable situation for the Romans, but one they could hope to erode through the pull of Romanization over time.