From 1973 until 1986 Armstrong was musical director of the Welsh National Opera. In this period he worked in collaboration with many leading European directors, including Peter Stein, Joachim Herz, Lucian Pintilie, and Harry Kupfer, conducting a wide repertoire of Verdi, Wagner, Janáček, Strauss, Berg and Britten and, in 1986, performances of the complete Ring Cycle. He has returned regularly to WNO as a guest conductor, notably for the world première of Peter Maxwell Davies’s "The Doctor of Myddfai", and for Peter Stein’s 1988 production of "Falstaff", which he also conducted in New York, Milan, Paris and Tokyo.