The Varusschlacht Museum at Kalkriese interprets one of the most famous defeats in Roman history: the destruction of three legions in the Teutoburg Forest in AD 9. Archaeology has made the site far more than a legend, with finds that help explain the battlefield and the Roman military presence in Germania.
It is outside Osnabrück, but it belongs in the city’s orbit as a major regional sight. The visit combines indoor exhibits with an outdoor landscape trail, so allow time for both. Even if ancient history is not your usual focus, the museum is good at showing how a historical argument is built from terrain, metalwork, and fragments.