Battle in the Teutoburg Forest (Latin Saltus Teutoburgiensis): the defeat of the Roman commander Publius Quintilius Varus against the Germanic tribesmen of the Cheruscian leader Arminius in 9 CE. In this battle, three legions (XVII, XVIII, XIX) were annihilated.
Written sources: the authors
There are four authors whose accounts of the battle in the Teutoburg Forest survives. Their value is questionable because none of them was an eyewitness of the Roman defeat, but they all use older sources which can be shown to be (near-)contemporary.
The oldest author dealing with the Teutoburger Forest disaster is Velleius Paterculus, a Roman officer and a personal friend of Tiberius, the general who had been active in Germania and was to become emperor in 14. Paterculus' account can be found here.
Paterculus had lived in the country east of the Rhine and knew many people who had been involved in the Germanic wars, including the Germanic leader Arminius and the Roman supreme commander Publius Quinctilius Varus, a member of Rome's highest nobility. Paterculus wrote his Roman History dedicated to Marcus Vinicius twenty years after the disaster, and must have spoken to survivors.
It is often said that Paterculus' account is biased because he wants to praise the deeds of Tiberius, but this is only a part of the truth. It is true that his loyalty to his former comrade-in-arms often degenerates to an almost disgusting flattery, but on the other hand, Paterculus is very outspoken about one thing: the soldiers of the three lost legions were not to be blamed for the catastrophe, and the battle was lost because Varus was an inexperienced commander.
This will not have amused Tiberius. Marcus Vinicius, to whom the Roman History is dedicated, must have understood that the people who were really responsible were those who had appointed Varus: Augustus and his right-hand man Tiberius. The latter had by marriage been connected to Varus, favored the Roman nobility anyhow, and was inclined to support claims that the legionaries had been cowards and Varus was not to be blamed. The Roman History of Paterculus made the emperor's position untenable.
Paterculus can only have come to this implied criticism of Tiberius, because he had information from people that he really trusted: survivors of the battle in the Teutoburg Forest. His account must therefore be taken very seriously.
The second source is the Roman historian Cornelius Tacitus, whose Annals were published at the beginning of the reign of Hadrian (117-138). (Tacitus' account can be read here.) Tacitus is a brilliant author, and, like most Greek and Roman historians, a moralist. In his view, the monarchy of the emperor Augustus created peace and a new type of politics, in which court factions secretly fought against each other. From this literary perspective, his description of the reign of Tiberius is a fascinating masterpiece (which does not mean that it is historically accurate): the emperor's powerful mother Livia, his ambitious daughter-in-law Livilla, and the sinister praetorian prefect Seianus are all involved in a struggle for influence.
The only hero is prince Germanicus, Tiberius' nephew, who stays away from intriguing and is portrayed as a successful young general, whose victories in Germania are important. This is almost certainly incorrect. After the battle in the Teutoburg Forest, Tiberius had led several punitive raids, but he had decided to give up the territories on the east bank of the Rhine. The Germanic tribes, left alone, would become divided again and cease to be dangerous. Germanicus resumed warfare, and was not very successful.
In Tacitus' Annals, Tiberius and Germanicus are opposites: the evil emperor and the noble prince. The description of the battlefield in the Teutoburg Forest is a case in point. The historian describes how Germanicus visits the site and buries the dead; this is in implicit contrast with Tiberius, who had had the opportunity, but did not do his duty. Because Tacitus tries to present Germanicus as a pious and courageous man, his description of the visit to the Teutoburg Forest is, compared to the importance of the event, too long, and the hazards are exaggerated.
Yet, he mentions several important details about the battle and there is no need to suspect that he invented them. It is almost certain that the source of his information was Pliny the Elder's History of the Germanic wars. We know for certain that Pliny, himself a military man and well-acquianted with the Rhine frontier, interviewed former legionaries that had been taken captive in the Teutoburg Forest and were liberated by Pliny's army in 50/51. Tacitus' description of the battlefield, which goes back to eyewitnesses, can therefore not be ignored.
Our third source is Publius Annius Florus, a contemporary of Tacitus who published an Epitome of the History of Titus Livy at the end of the reign of Hadrian. It is not a great piece of literature, but offers more than the title promises. After the excerpt from Livy, Florus continues his treatise with accounts of the wars of the empire, which include the Germanic wars of Drusus and Varus.
The description of the battle in the Teutoburg Forest (which can be read here) is spoilt by rhetorical effects, but contains two very interesting details. In the first place, Florus blames the emperor Augustus. This suggests that his account is not based on imperial propaganda. In the second place, he claims that one of the three lost legionary standards ('eagles') was still lost, and explains this with a story about a standard bearer who concealed the object in one of the marshes. This piece of information is corroborated by the Greek author Crinagoras (c.70 BCE - after 11 CE), whose poem on a man named Arrius survives in the Anthologia Palatina (7.741) and mentions how the heroic soldier secured a military standard.
If the incident is historical, it must go back to a Roman eyewitness, who did not see what happened next: how the Germanic warriors found the eagle. This certainly happened, because all three standards were later recovered by Roman armies. According to Cassius Dio, the last eagle was found among the Chauci in 41 (Roman History, 60.8.7). Unless we believe that Dio is wrong, we are forced to conclude that Florus' source was written after the capture of the second and before the recovery of the third standard. In other words, between 17 and 40.
The youngest and most valuable author is Cassius Dio (164-c.235), a Greek by birth, a Roman by conviction, and one of the greatest historians of Antiquity. His account is here.
Dio became a senator during the reign of Commodus, was made consul by Septimius Severus (204), served as governor in Africa and Pannonia Superior, and had the rare distinction of being made consul for a second time, together with the emperor Severus Alexander (229). Dio started his literary activity in the 190s and wrote his Roman History in 211-233.
Although Dio has, compared to Tacitus, a poor style, he is a superior historian, and this presents us with a problem: he has read very much, has a mind of his own, and presents us with a highly original account in which all information is used. This makes it impossible to find out which sources he used, unless we happen to possess the original source. But this is rarely the case.
However, we know that Cassius Dio was very careful in his use of his sources, and chose them no less carefully. For example, we can check his account of Julius Caesar's war in Gaul with the original, contemporary source (Caesar's own war commentaries). It turns out that Dio's excerpt remains pretty close to the original story. Of course, he adds some color of his own, but his main virtue is that he intelligently sees through Caesar's propaganda. His outline of the story is reliable.
We do not know which source Dio used for his description of the battle in the Teutoburg Forest, but we can be confident that he excerpted it carefully.
On the other hand, we must allow (as we must always do) for the possibility that Dio did not fully understand what he read. For example, like all ancient authors, Dio thought that the Germanic tribes lived on the edges of the earth, which the ancients conceptualized as a big forest, occupied by barbarian savages. Dio mentions mountains, ravines, and impenetrable forests. The battlefield has been discovered near Osnabrück, and there were neither mountains nor ravines. There may have been a forest, but it was certainly not impenetrable, because there was a village on walking distance from the excavated part of the battlefield. Yet, if we ignore his geographical bias, Dio is a reliable author.