By simply being more advanced, thanks to the switch to bronze. The dawn of these tribes happened during the period that humans were permanently moving away from their nomadic life in many places. The European tribes had dogs, horses, wheels and a relatively complex language, which gave them a great advantage over all tribes that were still stuck in the stone age.
Compare it to the European conquest of the Americas. Stone age nomads were no match to the first armies of organised groups of people.
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