6. The Battle of the Teutoburg Forest (9 CE)
In 9 CE, a coalition of Germanic tribes, led by Arminius, a former Roman officer, ambushed and destroyed three Roman legions under Publius Quinctilius Varus in the Teutoburg Forest. The Romans, unprepared for such an attack, suffered catastrophic losses, with estimates of 15,000 to 20,000 soldiers killed. This defeat halted Roman expansion into Germania and was a significant blow to Rome’s military reputation (britannica.com).
