One thousand nine hundred ninety eight years and a few months ago, the three-day-long Battle of Teutoburg forest ended on September 11th, 9 A.D.
During the course of those three days, Legions XVII, XVIII and XIX were virtually wiped out by Germanic tribesmen led by Armenius, a German chieftan who had been raised as a hostage in Rome and received Roman military training.
The Roman leader in the area was Quintilius Varus, appointed governor of what was to be the new province of Germania. He set about exacting tribute from the Germanic tribes and trying to build up the infrastructure of a Roman province as quickly as possible -- generating no small amount of bad feeling among his intended subjects in the process. Armenius was actually in his train for much of this time, serving as a leader among the Germanic auxilary troops.
Reports were brought to Varus that there was an uprising some distance to the north, and he set out with three legions to quell it. Their route took them through heavy forest, and since Varus did not believe they were yet in "enemy territory" the legion was strung out along the road, completely out of fighting order, and with engineering squads clearing trees and building bridges.
Armenius excused himself, saying that he was going to raise native troops to help, and vanished.
Not long afterwards, Germanic tribesmen attacked the legions all along their drawn out marching line. There was utter confusion in the ranks, and the Romans suffered heavy casualties, but eventually managed to draw themselves together and set up a fortified camp for the night. There they burned the equipment they couldn't easily carry on in battle order, and the next day succeeded in breaking out from their surrounded position, although suffering heavy losses.
They made some progress trying to escape over open ground, but then were forced to pass through another forested area where the Germans attacked again in even greater numbers. A heavy rain began to fall, making the legionaries' bows and javelins useless, and soaking their sheilds, making them heavier and harder to use.
Still they continued on, but on the third day marched into another ambush that Armenius has laid for them at the foot of Kalkriese Hill. There was a road around the hill on which the Romans were able to move forward in good order, with the hill on one side and swampland on the other. The Germans had thrown up a defensive wall across this road, from the hill to the bog, and thus had the Romans trapped in a dead end. The romans tried several times to break through the rampart, but failed.
At this point Varus and a number of the other officers took the Old Roman way out and fell on their swords. Thousands died fighting, a number of captured officers appear to have been killed in sacrifices to the Germanic gods, and captured common soldiers were made slaves. Some 25,000 men were killed or captured -- the greatest Roman defeat since they fought Hannibal.
Suetonius records that when Augustus received news of the catastrophic defeat, he had a mental breakdown and went wandering through the palace pounding his head against the walls and crying "Quintilius Varus, give me back my legions!"
The setback was so major that it ended Roman expansion in that direction. The rest of Germany was never conquered by the Romans.
Forty years later, around 50 A.D., Tacitus tells how when Roman legions defeated an invading group of German tribesmen, they found among them and liberated survivors of the battle who had been in slavery for forty years.
Should you want to read about it in depth, the standard modern work on the topic is Adrian Murdoch's Rome's Greatest Defeat: Massacre in the Teutoburg Forest.
You can also read Dio Cassius' account online here, starting in section 18.