The Germanic tribes spent many hundreds of years after Rome fell unable to agree on much of anything. They wared with each other in a seemingly never ending cycle. Then they started to suffer Zarmatian raids, helped deal with the Huns then later, they felt the heavy influence of Kymbri and their allies.
Using the word "tribe" might give one the wrong impression. It is not meant to imply that they were primitive, far from it. They were actually a dozen small states who were on the verge of absorbing each other. They had cities, military forts, and many important ports in the the north along with some of the most important trade routes. By all accounts, they should have had enough power to make Kymbri itself sweat, but the complicated, messy local politics did not bring their potential to fruition. 

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The Germanica War
The border between the tribes and Gallia was pushed back and forth throughout the years. Atrocities were committed. Both sides blamed each other and requested aid. Gallia requested aid from Kymbri, while the Germanic tribes started to come together for a common purpose. When Kymbri refused to give aide. The tribes felt emboldened and invaded the border in force in hopes of putting Gallia into an inferior position at the negotiating table.
During that time, Kymbri was embroiled in a civil war. So they could do nothing. Instead they asked Eire and a few western regiments in Alba(creating the Celti-pact) to intervene and bolster the Gallian army. After many battles, the tribes were pushed back to their original borders. By then, the civil war in Kymbri was over and they officially joined the fight, taking over Germanic territory until each tribe surrendered. Kymbri saw the need for another big state to protect against Zarmatia and Mithras, but were uninterested in helping to implement a new government there. Eire took over this task and decided to humiliate the tribes further by naming the area Gaelland.

The Humiliation
The humiliation lasted for over 400 years. Few can claim a lack of Gaelic, or Gaulish-Romano blood, but the defeat was never forgotten and despite the tribes being considered second class citizens and heavily discriminated against, tribal identity is stronger than ever. Cultural frustrations culminated in the city of Turicia, where an uprising of Rhaeti took the city for themselves. More followed suit. This helped to encourage the Mithrans to fly over Gaelland to directly attack Kymbri, starting the first Mithran war.