Battle of the Teutoburg Forest
The Battle of the Teutoburg Forest was a decisive defeat of the Roman Empire by a group of Germanic tribes commanded by Arminius. The Roman commander, Publius Quinctilius Varus, committed suicide to avoid capture.
Modern military significance
The victors kept up relentless pressure of raids and ambushes. Their tactics have been described as an early version of swarming.
It also is a demonstration of asymmetrical warfare, in which the Romans were manipulated, by what we would today call the operational art of Arminius, into not fighting from their preferred tight formations and fortifications.