Romans regularly intervened in Germania to extract tributes, obedience or unequal treaties. In the Ist century, in spite of the defeat of Teutoburg and giving up on directly controlling parts of Germania, Romans were still able to clientelize various confederations and people across the Rhine and the Danube : how imperial influence was able to take place was a case-by-case situation, but could cover forced appointment of kings, enforced military or economic treaties, forced migrations, etc. but for the larger part of imperial history, Romans were able to intervene politically and militarily in their "near abroad" in the Barbaricum, even late into the IVth century trough subsides or religious patronage, etc.

In the Barbarian world itself, the constitution of confederations trough more or less dependent relations wasn't unheard of : Ariovist's coalition spearheaded by Suebi, Hunnic hegemony over Germanic or Sarmatian peoples, Marcomannic leadership on the Upper Danube, etc. As these weren't as much based on strong hierarchical and administrative structures, comparatively to the Empire, they also were more unstable, more negotiated or dependent on rulership and eventually proner to fragment; but hierarchical domination of and on Barbarian peoples were a staple of Barbarian politics, something Romans tried to keep the upper hand on but did not actively created out of the blue.

In the Vth century, literary and archaeological sources can point to these relations : Ostrogoths holding Thuringians as clients and allies against Franks, Gepids under Ostrogothic or Byzantine overlordship, Rugii or Herulii being expelled or swallowed up, etc. and, notably, early Merovingian kings under Hunnic or Byzantine clientele. In spite of Clovis supremacy over Rheinish petty-kings and his victories against Alamans, Franks do not appear particularly powerful in northern Europe before the mid-VIth century : Frankish victories against further raids, such as the Danish defeat in 516, but also the collapse of Ostrogothic influence over Central Europe with the Gothic wars, gave Merovingians the opportunity to settle themselves as the prime power in western Europe, especially with Theudeuric and the line of Messine kings, who strived and managed to fight and submit Thuringians (former Gothic clients) and neighbouring peoples.

As Francia appeared as the most prosperous and powerful polity of post-imperial western Europe and as kings in Italy were unable to really preserve Theodoric's diplomatic network, Franks were either a necessary partner or a powerful threat : the first Frankish intervention in Thuringia (with Merovingian kings probably had genealogical relations with) was even made at the behalf of a Thuringian king against another, hoping to get half of it which they did after an obscure situation where the supported candidate died. It's not that they annexed all of Thuringia : a good part was eventually swallowed up by neighbouring entities, Germanic or Slavic, and the land within the realm was probably let to local nobles, poorly settled at best by Franks, with local populations being held tributary.

Indeed, while both Gaul and Germania were part of a same Frankish realm (regardless if it was under a sole "monarch" or various kings), they weren't ruled the same way, either politically or administratively. The Merovingians kings ruled over the 'Roman' parts of the realm, essentially Gaul, as both a 'king' over a militarized aristocracy and a 'princeps' i.e. the top of a light-weighted imperial state. Both of these aspects were fused on a same person and participated of a same rulership : but in regions where either Roman social and institutional structures (urbanisation, provincialisation, religious network, etc.) either disappeared or never really took place to begin with, the military and warlord aspects had a much greater importance in their dealing with peripheral peoples, especially (as for Roman emperors) their capacity to halt raids and project their power beyond the Rhine themselves.

Submitted people in Germania weren't "provincialized" or firmly assimilated to the late Roman state as it existed in Gaul (even in Gaul itself, peripheral peoples as Saxons in modern Normandy or Picardy, Bretons or Wascons, weren't) but as autonomous entities until the Carolingian period, although they were held tributaries, saw their rulers being appointed by the Merovingian court (that maintained a semi-fiction of identifying as "counts" or "dukes" even when they styled themselves as kings), owed military service, enforced matrimonial and dynastic relations, and critically enforced a personal relation between the Frankish king as overlord and the peripheral duke as vassal.

It could give the impression that Frankish Germania was kind of an afterthought, but it might well be the contrary : tributes paid in cattle, horses, possibly slaves or levied men served as auxiliaries in Frankish campaigns, all of that participated to royal prestige, wealth and "shielding" against further Barbarians or rivals within or outside the realm.

But Frankish Germania wasn't much of clearly formalized geopolitical entity and the difference between a people submitted to Frankish overlordship as Thuringians and a people periodically tributary as Saxons isn't necessarily obvious : the capacity to "hold out" Frankish power and not seeing themselves included into Merovingian formal overlordship by having them appointing Frankish rulers on them is probably the most important in drawing a line between an "outer" and "inner" sphere of hegemony. Further difference, for being integrated within a formal hierarchical relation with the Frankish kings, either vassalic or matrimonial, and thus within the Frankish realm was expectations of obedience not just on obligations but in forms of power : its apparatus, its participation to the realm affairs, and by the VIth century, expectations of Christianisation, adoption of Frankish customs and redaction of law codes on the Salic model (as the Alemannic Law under Dagobert) precisely to bind these rulers and their duchies more tightly.

Of course, this was true when the Merovingian kings were able to enforce their rule beyond their borders, bullying their way into submission of local kings, as Theuderic I and his sons did, but could at the first sign of weakness be fought against : Clothar I had to campaign against rebellious Saxons; Clothar II and Dagobert I had to fight their way to re-establish Frankish overlordship after that peripheral dukes gained significant independence during the royal faida and civil war in Gaul.

Finally,the Late Merovingian crisis was the nail in the coffin of Frankish Germania, with Thuringians, Alamans or Bavarians essentially acting independently, barely acknowledging a remote Merovingian suzerainty if still partaking in Frankish aristocratic and matrimonial networks; something Carolingians efficiently, but brutally, dealt with.

It's not so much Franks succeeded where Romans failed than either having different focuses and interests on the region : Merovingians, at least at first, did not greatly different from Imperial policies in Germania in setting up unequal, tributary relationship with Barbarians. The difference, in that these relations were translated into forms of control could resides in the double nature of the Frankish kings, as both princeps and king and able to create strong dynastical and vassalic ties over German peoples; as much as being spared much outer threats or pressure on their other borders. But as much it was transformative, in that it resulted into a Frankish influence and "Frenchification" of local aristocraties' culture and own networks, it shouldn't be taken for a sheer conquest or political assimilation.