Complex sentence: His first operetta was "Indigo und die vierzig Räuber" (1871), and his third operetta, "Die Fledermaus" (1874), became the most performed operetta in the world and remains his most popular stage work.
Split into simpler sentences: His first operetta was "Indigo und die vierzig Räuber" (1871). His third operetta, "Die Fledermaus" (1874), became the most performed operetta in the world, and remains his most popular stage work.