The Battle of Ruuria, also known as the Ruurian massacre, was, to put it simply, a decimation of the Ruurian race.
Once on Ruuria the Sith found themselves stalled by the native Ruurians, whose refusal to engage the Sith in open battle meant a much more prolonged conflict that the Sith had hoped for.
Although the Galactic Alliance had a small contingent of troops on Ruuria, the planets strategic position was ambiguous at best, and the true reason for Ruuria's invasion was unknown until a few months later.