The Helvetii weren't German, they were Celts from the Alps. They were migrating due to pressure from Germans. The movement was planned by the Helveti king Orgetorix, who died before the migration began, while raising an army to fight a revolt. The revolt was spawned by his conspiring with Dumnorix and Casticus to split Gaul between the three of them; the people didn't like the plan, and plotted to arrest him, try him, and have him executed if found guilty. He discovered that, rose an army of 10,000 men, but died while they were massing, likely due to suicide. The Helvetii went ahead with the migratory plan though.
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