Cohn made his reputation helping secure the death penalty for accused Soviet spies Ethel and Julius Rosenberg (one of his proudest moments, according to his autobiography), which won him a job as counsel for Senator Joseph McCarthy, the infamous politician who shot to national fame during the Communist witch hunts of the 1950s, spawning the eponymous term McCarthyism. Unencumbered by the backlash that effectively ended McCarthy's career, Cohn returned to New York, where he forged a path as the kind of morality-free, dogged powerbroker Trump needed to guide and protect him through the minefield of New York City politics.
Marie Brenner's Vanity Fair feature on the Trump-Cohn connection is far more interesting, and takes far less of your time, than watching even the first (and most compelling) installment of An American Dream. Cohn knew everybody from newscasters to politicians to mob bosses, and could pull strings for anyone he deemed worthy, including Trump, whom he at one point called his best friend. Having helped Trump secure the rights to the Commodore/Grand Hyatt, Cohn also advised him on the construction of the first property Trump built from the ground up: Trump Tower.