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An alliance of German tribes led by Hermann the Cherusci [known by the Romans as Arminius] a Roman trained soldier, and, I believe a Roman citizen, ambushed three legions [XVII, XVIII, and XIX] under the extremely inept command of Quintillius Varus, and annihilated them [and their families and camp followers in a three day running battle in the Teutoburg Forest. [Hermann led them into the trap].Aside from punitive expeditions, and a mission to recover the lost eagles, and properly bury the dead, the Romans pulled back to the Rhine, and never sought to annex Germany east of that river again [although archaeologists are now finding a fairly extensive net of German-Roman settlements from before the battle. the three legions were never reconstituted.

Although not 'Euros', the peoples with the best track records against the Romans were the Carthaginians [Hannibal KILLED 50,000 Romans at Cannae], the Parthians almost entirely annihilated Crassus' army at Carrhae, and the Sassanid Persians [who actually captured a Caesar]. Second honors would go to the Huns and the Goths [Adrianople], but the Roman Army they faced was not the Roman Army of earlier times.