Operetta
In the early 20th century the French operetta became less popular as there was more interest in the Viennese operetta.  After World War I Berlin was the centre of German operetta.  By the middle of the 20th century many composers were writing light operas which were based on American dance band music.  These were no longer called “operettas”.  The word “operetta” continued to be used for light operas which were more in the style of traditional central European music.