@Camazotz: Here is what happened.

Augustus Caesar, the first emperor of Rome (before him Rome was ruled over by elected officials) advanced Northward to try to capture Germanic tribes in what is today considered Germany. At the time, Germany existed in two halves, a Western half and an Eastern half, and they were both governed separately. The Western half was fairly easy for Rome to conquer, but they would soon have the exact opposite opinion of the Eastern half.

As they approached, the commander of the Eastern half, who it turns out had crept into Rome to eavesdrop his way into getting a Roman education in war and snuck out some weapons before returning, saw them and lured the to-be battle into Teutoburg forest, a large forest in central Germany. Rome had put all their eggs in one basket, and after an exchange between the Romans and Eastern Germans where the Romans took themselves for granted and forgot they weren't sticking around just to flip people off, out of the woodwork came the most effective Rambo-style fighting force in the world at the time which took advantage of the fact everyone was drawn in, surrounding the Roman vicinity and inflicting its highest military loss on them. Roman casualties? A few dozen thousand. German casualties? Err, well, a dozen. To put it into perspective, even the very comparable Spanish battle of Tenochtitlan was more balanced.

The Roman loss was enough to stunt its growth like a clipped tree, and that part of their military was never even rebuilt, similar to how Mongolia refused to rebuild its navy following their defeat by the Japanese or the Japanese not wanting to rebuild theirs after losing to the US in WWII, because they thought the legion numbers were cursed. That's how bad the outcome turned out. The battle even became a taboo topic in Rome, especially considering one of the casualties was their own general, whose head the Germanic tribes kept as a trophy, and considering the Roman emperor Augustus was considered the best of all past and future rulers. It being a sensitive topic in Rome, it was always fresh on their minds when Germanic tribes were literally busy existing.