Wozzeck
Though Berg began work on the opera in 1914, he was serving in the army during World War I, so he did not have time to continue working on it until 1917.  He finished the opera in April 1922.  Erich Kleiber conducted the first performance at the Berlin State Opera on December 14, 1925. It quickly became famous and was performed in all of the big opera houses in Europe.  It was so successful that Berg was able to live off of the money he earned from it.