@Duncan

Teutoburg Wald was a vast ambush, not a set-piece battle. No (Roman) commander would choose to fight there. The Germans chose to ambush the Romans on the road they were using through the woods specifically because they knew it would disrupt the ability of the Romans to establish command and control. This was one of the things Arminius learned from the Romans: We don't fight where we can't see our own army. The only reason Varus was there is he (apparently) trusted Arminius to lead him safely through what was becoming increasingly dangerous territory. I guess there's another lesson the Romans taught Arminius: Backstabbing is much easier if they trust you to watch their back!