How could a bunch of "uncivilized barbarians" defeat one of history's most organized and well trained armys?
Arminius was a trusted adviser to Varus, who then led Varus and his legions into a trap, as Arminius was actually allied with a number of Germanic tribes.
The Roman army was disciplined and well equipped and could be very powerful. However there were instances where strategically and tactically there was little they could do as their leaders brought them into hopeless situations, namely the Teutoburg Forest, or Carrhae years before.
Command and control immediately broke down as the Romans were hit simultaneously and unexpectedly from both sides by a determined and aggressive 'uncivilized' opponent. It was a rout, and a massacre.
Arminius was a clever opponent, and his duplicity ensured a very high percentage chance of victory. Well educated in Roman tactics, being an almost double agent of sorts, while also having almost complete control over the conditions leading up to the battle, he was able to draw the Legions away from each other and defeat them with locally superior numbers.
In essence, the Romans stood no chance. The tribesmen may have been 'uncivilized' but the tactics employed were very effective.
Source: The Battle that Stopped Rome: Emperor Augustus, Arminius, and the Slaughter of the Legions in the Teutoburg Forest Book by Peter Wells
THe answer given is hopelessly out of date. The battle is now believed to have taken place much further north near Osnabruck, at a town called Kalkriese as I recall. This was because several years ago evidence of a large battle was found there and the dating of coins found in the area strongly indicates this was the battle in question.
It seems to have been a running battle with the Romans being broken up into smaller formations and trying to defend themselves in those circumstances. The story I read suggest that this was part of some planned retreat and not some trap.
I will try to locate that story.
EDIT: this link is to the museum there and all the recent news: