大败 is an auto-antonym (反义同形词): the same verb can mean either 'to utterly defeat (someone)' or 'to suffer a crushing defeat'. The meaning depends on the grammatical structure — when followed by a direct object, 大败 means 'defeat (the object)'; when used without an object or in a passive context, it means 'be defeated'. This is a high-level vocabulary item that requires careful attention to sentence structure.