Complex sentence: He was on a two-week mission to find a black composer to collaborate with him on a folk opera and was armed with a letter of introduction to José Antonio Fernández de Castro, his door to Cuba's artistic world.
Split into simpler sentences: He was on a two-week mission to find a black composer to collaborate on a folk opera. He had been given a letter of introduction to José Antonio Fernández de Castro, his door to Cuba's artistic world.