Why is Alex actually being deported from the United States in the novel, Leaving Berlin?

Alex, a successful novelist living in Hollywood, whose associations with suspected communists in the film industry brings him face-to-face with the House Un-American Activities Committee. His refusal to cooperate gets him deported, but what the Congressmen, his estranged wife and his 10-year-old son do not know is that the deportation has become nothing more than a cover story. In a desperate effort to get his life back and see his son again, Alex has made a secret agreement with the U.S. intelligence agency.