The relationship between the “barbarians” and the Roman Empire was always rather complicated, even going back to the days of the Republic, but I’ll mostly focus on late antiquity here since that’s what I’m more well-versed in. But first I think it should be said that the “Germanic” people’s you speak of should not really be conflated with the Holy Roman Empire. In fact, the term “Germanic” is somewhat problematic because the Goths, Vandals, Suevi, Franks, etc. never considered themselves to be a related group of peoples. Most of these groups were actually different tribes that the Romans grouped together under ethnic identities like Goths (there were actually Thervingi and Greuthngi Goths and then of course the Vesi and Ostrogoths), and then further grouped them together with groups like the Vandals as “Germanic”. These peoples might all have spoken Germanic languages, which may have been related (I’m not a linguist so if this is wrong somebody please correct me) and there may have been some cultural overlap in places but that’s about it. More historians are starting to avoid using terms like Germanic, or at least provide context like I have, because we now know better. But with that having been said, on to the actual question.

To truly answer this question, I have to give a decent amount of background information to truly put things into context, so bear with me for a while. As I already stated this relationship was very complicated and there was quite a bit of cultural exchange between Romans and non-Romans, especially in late antiquity as more and more non-Romans moved into the Empire. I think it is inaccurate to say that these peoples were “constantly” fighting the Romans. Yes there was plenty of conflict along the borders but a lot of this conflict was raiding conducted by non-Romans and then the Romans retaliating in attempt to get the raids under control. But the tribes closest to the Roman borders also depended heavily upon the Empire. In his book Barbarian Migrations and the Roman West (which unfortunately I cannot cite as I used a library copy of the book for MA research), Guy Halsall discussed the exchange of goods between Romans and non-Romans. The barbarians (a word I only use to avoid saying non-Roman over and over) who lived across the Rhine would often seek out gifts from Roman emperors and in turn would use the items that they received to show off their wealth and status and maintain control over their people. This was beneficial for Rome in multiple ways as it allowed them to have influence over who had power near the borders and helped curtail raids into Roman territories. This also meant that the Romans could just as easily destabilize the power base of a given region by no longer offering gifts or by offering them to a different person/group. If one tribe was largely able to maintain its dominance in a region because of the implied support, they had from Rome then removing this implication could delegitimize them in the eyes of others. Halsall is also one of the historians, along with Walter Goffart, who argues against the idea of the so-called “Germanic” people having an established shared cultural identity, which is a view that I tend to be in agreement with.

Of course, in some cases this same practice could, and did, backfire on the Romans. The most prominent example is probably that of the Goths. Emperor Valens led campaigns into Gothic territory in 367, 368, and 369 (though the one in 369 was the only one that really succeeded), and this already caused problems among the Goths as the peace treaty they signed mostly favored the Romans. Any issues caused by the peace treaty were compounded by the fact that Athanaric, a Gothic king or chieftain, began to persecute those of his people who practiced Christianity, which may have been a ploy on his part to reassert his control following such a disastrous defeat. The reason I say this may have backfired on the Romans is because, in addition to the treaty, they might have been using Christianity to gain the trust of Frittigern, who led the faction opposed to Athanaric. This opposition eventually led to a war between the two factions with quickly destabilized the region even further. Socrates Scholasticus gives us a pretty good insight into the conflict between Athanaric and Fritigen, and explicitly states that Valens sent an army to help the latter (though he claims the conversion to Christianity came after this rather than before):

When the latter [Athanaric] obtained an evident advantage over his rival, Fritigernes had recourse to the Romans and implored their assistance…this was reported to the Emperor Valens, and he ordered the troops…in Thrace to assist those barbarians who had appealed to him against the more powerful countrymen, and by means of this subsidy they won a complete victory over Athanaric…totally routing the enemy. This became the occasion for the conversion…to the Christian religion…Fritigenres…embraced the religion of his benefactor, and urged those under his authority to do the same. Therefore it is…the Goths are…infected with the errors of Arianism. (Socrates Scholasticus, Church History, 4.33)

While sending an entire army across the Danube to support Fritigern is well beyond the scope of sending gifts as a form of implied support, it still fits within the pattern of the Romans using their influence to determine who would be in control of specific areas. Its also important to examine the fact that Valens chose to help Fritigern in his struggle over Athanaric rather than simply letting the latter emerge victorious. It could be that Valens simply wanted to inflict further defeat upon Athanaric, but it seems more likely that he believed Fritigern would be more friendly to Roman interests in the region. The fact that Fritigern converted to Christianity, and more specifically Arianism, implies that Valens already knew Christianity played a role in the conflict and that Fritigern’s group of Thervingi was already open to the idea of Christianity. This also meant that Fritigern’s entire basis for power already relied solely upon Valens and remaining in his favor, because he was only able to maintain power thanks to the Romans. It should also be noted that even though Socrates says this was a complete victory he may have been overstating things as Athanaric remained the leader of his faction of the Tervingi. Its more likely that the Christian Goths were now able to split off from the rest of the Thervingi with Fritigern as their king.

It was during this conflict that the Huns appeared on the scene, first defeating and subjugating the Alans (whom Ammianus refers to as “friends” of the Huns). The Huns next defeated the Greuthungi Goths, who’s king then committed suicide, before defeating Athanaric’s faction of the Tervingi in battle. At this point the rest of the Tervingi led by Fritigern and Alavivus decided to flee and migrated towards the Danube river and asked to be allowed into the Empire, likely as dediticii. Fritigern likely believed Valens would allow them entrance into the Empire because he had already supported them once before and because they were not only already Christians, but also followed the teachings of Arian.

This is where things started to roll downhill and is probably why so many people view the non-Romans of this period (especially the Goths) as being hostile or antagonistic towards the Roman Empire. For starters, Valens was in Antioch at the time as he was campaigning against the Persians, this means it would have taken much longer for messages to be sent back and forth and the Goths were probably stuck on their side of the Danube for a longer period of time than Fritigern had hoped. They likely were not able to carry very much food with them either, not to mention they would have needed to eat while migrating towards the border, and we know the Romans had to feed them once they crossed the border. So by the time the Goths were finally allowed into the Empire they were probably extremely hungry, exhausted, and likely not very pleased about having to leave their homes behind. According to Ammianus once the Goths were allowed to cross they faced further food shortages and harsh conditions imposed on them by the Romans which only angered them further. Ammianus would have us believe that these food shortages were intentional because the Romans withheld food from the Goths to drive up the prices, but it seems unlikely that Valens would be ok with local officials doing this to people who hoped to recruit into his military, so I tend to believe that Ammianus exaggerated some of the corruption involved with the Goths receptio.