Arminius, leader of the Cherusci tribe and former Roman auxiliary commander, brought to Varus the news that there was a huge uprising of German tribes hostile to Rome. While cetainly unwelcome, this was not a huge surprise -- Rome had been fighting fires on the frontier of Germania since Julius Caeser created it after conquering Gaul. So Varus set forth to suppress the rebellion with the XVII, XIIX, and XIX'th legions; following behind was a massive baggage train, stretching to as much as 15km, with food, materiel, household goods etc., and accompanied by servants, wives, children and assorted other camp followers.
In the region of Kalkreise, 20km NE of modern-day Osnabrueck, Germany, the entire troop was spread out along several kilometers of a narrow track of solid land between a looming mountain (OK, a large hill to those of us from Colorado and Switzerland) on one side and a vast marsh on the other side. In some places the solid ground was hardly wide enough for the carts. While the invading force was spread out in ths fashion, unable to keep in formation due to the terrain, they were ambushed by the waiting Germans. For several weeks (estimated by the amount of work involved) huge blinds were constructed up the base of the mountain along this winding track. This wall of dirt and foilage served two purposes -- it further funneled the Romans into an attenuated line, and it supplied a hiding place for thousands of attackers.
Imagine -- it's cold, overcast, your weapons are slung, you are expecting no trouble, and are bone tired from days of marching. Then while your head is down, slogging forward in single file behind the equally tired soldier in front of you, you hear a tremendous yell and the next thing you know spears are being launched at you from what appears to be uninhabited, virgin forest. Meanwhile, the kilometers-long train behind you doesn't know what's going on, and continues foward, creating a massive crush of beast and humans in the one path out of the slaughter. The only amazing thing is that it took three days of battle to mop up the fleeing fragments of the doomed party. Only a handful of survivors made it out. However, within five days of the defeat Caeser Augustus was informed of the worst military disaster to befall the Romans in centuries. Amazing, considering how long it takes mail to Rome today.