Arminius (Hermann) was a 28-year old Germanic aristocrat from the Cheruscans tribe. As a leader of Germanic auxiliary forces he had been serving Rome for a long time, and had even obtained Roman citizenship and the title of a Roman knight. Arminius had learned enough of the Roman art of warfare in order to know that he and his warriors would definitely fail in an open battle against the disciplined and well-equipped legionaries.
Therefore, in 9 A.D, he told Varus of an alleged rebellion and then made guides available to him. The guides were his loyal men to lure Varus into a trap.
As usual, Varus and his entire army were about to move to their winter camps. Along the way they intended to quickly strike down the rebellion. Though having been warned of betrayal by one of Arminius' relatives, he trusted his Germanic guides. So with three legions and an enormous baggage, totaling more than 20,000 men, he moved through a swampy, complex and unfamiliar forest area (probably north of today's German city of Osnabruck).
The soldiers were marching in comfortably free order with their helmets off and shields on their back, when they were suddenly ambushed by the men of Arminius who were Germanic and had formerly served in Roman auxiliary troops. Arminius' men had been equipped by the Roman arms and turned against their former allies and masters.
The fights stretched over more than three days. As for the legionaries, continuously strong rain affected visibility, silted up the ground and - along with cold weather - started to waste away their endurance. The slowly moving baggage prevented them from a fast withdrawal out of this adverse setting.
Above all, they were not able to carry out effective resistance because after each attack the Germanic units pulled back into the protective forest. While at first the Roman army had withdrawn in an orderly fashion, soon a general panic set in where everyone was only trying to save their own life.
What followed was a terrible slaughter: Some Romans would not even show resistance anymore, others would kill themselves, and most of them, disoriented and weakened, fractured into small groups - would be slain one by one by Arminius' warriors.
There were virtually no Roman survivors. Three of the best legions were not just defeated, but annihilated. One sixth of the entire armed forces of the Roman Empire was destroyed.
For 30 years the Romans had been arduously exploring the Germania. They had won over Germanic tribes as allies or had subdued them, they had built roads, they had established a province administration, they had founded cities. Over 50,000 men had been employed for three decades in an effort to civilize Germania and to make it part of the Roman empire, and had been paid for their work, equipped, accommodated and provisioned by the Roman state.
But ultimately more than 20,000 of their men had been killed within a few days. Now Germania was a province no more, but a free and hostile country.
Arminius had sent to Marbod Varus's head as a call to join the war against Rome, but he was satisfied with his own empire. He didn't want to take part in a risky war; so he sent Varus's head back to Emperor Augustus in Rome.
To the Romans' surprise and joy, the rebellious and victorious Germanic tribes did not attack Gaul or Italy. They seemed content with having regained their old way of living and their independence, and leading numerous small wars against each other. Actually, they were in no position to attack the Roman Empire.
The Germanic tribes in the north were still allied with Rome. In the west and south there were the legions of Roman Gaul and Rhetia, and in the east the Germanic kingdom of the Marcomanians was holding out, showing ominous neutrality. The Rhine river remained the border between Roman Gaul and the free Germania.
Arminius would not become king of a united Germania. Rather, he was murdered by his relatives, and his wife and son were captured and handed over to the Romans by Arminius' father-in-law. The outcome of the battle established the Rhine as the boundary of the Roman Empire for the next few hundred years, until the decline of the Roman influence in the West. The Roman Empire never was able to conquer Germania, although many attempts were made thereafter.
The battle had a lasting setback on the Roman attempt of taking over Germania which seriously started in 14 BC and had long term historical consequences as it set the boundary between Romance languages and Germanic languages.