Actually, Arminius was a Roman Knight born from German nobility, taken to Rome when he was a child, as a safety measure. He followed Varus back into Germania as a counsel. When Varus started to try to implement Roman laws and taxes in the controlled territories, the Germans tribes were not pleased. Arminius united some of the tribes in secrecy and then told Varus to take his army through a forest, because he knew the German armies would have a hard time winning a battle against the Romans in the open, where they could apply their tactics. He also correctly assumed that when attacked in the forest, the Romans would withdraw into the open, to the nearby river banks, and he fortified the river banks.
As was said later, he died due to an internal power struggle. Just because the Germans wouldn't be united by Rome, didn't mean they wanted to be united by Arminius.