What happened was that Arminius accompanied Varus during his summer stay in Germania in 9 AD, all the while secretly mobilising the Cherusci, Chatti, Bructeri and other tribes. When the Roman army marched all the way from the Weser back to their winter camp in the forts west of the Rhine, Arminius guided them away from the main way (Cassius Dio mentions a pretext that there was uprising, which would not explain why Varus took the entire train of noncombatants on a minor punitive expedition, though). Arminius then left the army while they cleared a way through a terrain of woods and bogs, and obviously under formidable German autumn rains, to spring the final trap.
I'm a writer of Historical Fiction and Fantasy living in Germany; literary science academic, historian, interested in archaeology, avid reader, opera enthusiast, rider, traveller with a liking for foreign languages, and photographer.