Complex sentence: The two remained friends after Szell left the Royal Court Opera in 1919; even after World War II, when Szell had settled in the United States, Strauss kept track of how his protégé was doing.
Split into simpler sentences: The two remained friends after Szell left the Royal Court Opera in 1919. Even after World War II when Szell had settled in the United States, Strauss kept track of how his protégé was doing.