Bessie held strong left-wing views and joined the International Brigades during the Spanish Civil War fought alongside Robert Merriman, David Doran, Leonard Lamb, Joe Bianca and Edwin Rolfe, who he described as being "frail; he resembled a bird; he had a fine, delicate bone structure and he did not look as though he should be in an army... I do not think I have ever met a gentler guy, a less pugnacious guy, less of a soldier. But he had the iron of conviction in him just the same. He had a tiny automatic pistol some one had given him, and it became him, though I could not imagine him ever using it."
While in Spain he was interviewed by journalists, Ernst Toller, Ernest Hemingway, Vincent Sheean and Herbert Matthews. Bessie took part in the battle for Gandesa and served under Milton Wolff at the Ebro. On his return he wrote a book, Men in Battle (1939) about his experiences in Spain. He was also appointed as drama and film reviewer of the New Masses (1939-43).
Bessie moved to Hollywood and two of his screenplays were produced by Warner Brothers, Northern Pursuit(1943) and The Very Thought of You (1944). His next screenplay, the extremely patriotic, Objective Burma (1945) was nominated for an Academy Award. This was followed by Hotel Berlin (1945), Ruthless (1948) and Smart Woman (1948).
After the Second World War the House of Un-American Activities Committee began an investigation into the Hollywood Motion Picture Industry. In September 1947, the HUAC interviewed 41 people who were working in Hollywood. These people attended voluntarily and became known as "friendly witnesses". During their interviews they named several people who they accused of holding left-wing views.
Bessie appeared before the HUAC on 28th October, 1947, but like, Herbert Biberman, Lester Cole, Albert Maltz, Adrian Scott, Dalton Trumbo, Edward Dmytryk, Ring Lardner Jr., Samuel Ornitz and John Howard Lawson, he refused to answer any questions. Known as the Hollywood Ten, they claimed that the 1st Amendment of the United States Constitution gave them the right to do this.
The House of Un-American Activities Committee and the courts during appeals disagreed and all were found guilty of contempt of Congress and Bessie was sentenced to twelve months in Texarkana Federal Correctional Institution and fined $1,000.
Blacklisted by the Hollywood studios, Bessie worked as a stage manager in San Francisco. Bessie, who left the American Communist Party in 1954, resumed writing and published several novels including The Un-Americans (1957), The Symbol (1966) and One For My Baby (1980). He also wrote Inquisition in Eden (1965), an account of his experiences with the HUAC.
Alvah Bessie died of a heart attack in Terra Linda, California, on 21st July, 1985. His son, Dan Bessie, directed a movie, Hard Travelling (1986), that was based on his father's novel, Bread and Stone.