Complex sentence: Diaghilev hired the young Stravinsky at a time when he was virtually unknown to compose the music for "The Firebird", after the composer Anatoly Lyadov proved unreliable, and this was instrumental in launching Stravinsky's career in Europe and the United States of America.
Split into simpler sentences: Diaghilev hired the young Stravinsky at a time when he was virtually unknown to compose the music for "The Firebird", after the composer Anatoly Lyadov proved unreliable. Diaghilev was instrumental in launching Stravinsky's career in Europe and the United States of America.