Investigations on suspected subversion have numbered a close second to war inquiries. But challenging the conduct of a war meant challenging the President (that's why most investigations have occurred after wars); challenging subversion, however was politically safe. Subversives, if they actually existed, were on the margin of society, and had few defenders. Members of Congress could foam-at-the-mouth with little worry of negative political consequences if they launched an investigation on suspected subversives.
During World War I, Congress investigated German subversion in the US. When Germany was defeated, the focus immediately shifted to Bolshevik subversion, which was the "1st Red Scare". The House Committee on Un-American Activities was created in 1937 (it was originally called the "Dies Committee" since it was under the leadership of Chairman Martin Dies, Jr.
D; TX, pictured above), and launched its first investigation in 1938 . . . Dies tried to prove that communists were linked to the New Deal. In 1946, the committee started using the acronym HUAC (House Un-American Activities Committee).
In the spring of 1947, HUAC (pictured; HUAC committee member, California Representative Richard Nixon) launched a new investigation and sent members to assess the situation in Hollywood; studio executives were questioned about FDR's influence in making WW II propaganda films in support of the USSR . . . but the investigation that spring was nothing compared to the all-out investigation in the fall.
During the fall hearings, studio executives named people, including Dalton Trumbo (pictured; screenwriter) by Jack Warner; Director Sam Wood ("Kings Row", in which Reagan was a star) also listed names, and Louis B. Mayer was cooperative. But Mayer told HUAC that the industry did a great job of keeping communists at bay; these executives walked a thin line, in that they didn't want Congress to censor movies, but the executives also didn't want to be seen as obstructing a high-profile Congressional investigation . . . especially by movie-goers.
Reagan echoed what was stated before, being more specific without identifying people. He stated under oath that he believed that there was a communist influence in Hollywood, based on the votes of the Screen Actors Guild (SAG; of which he was their President) on certain issues. Reagan went on to say that Hollywood could police itself, claiming that 90% of SAG was communist-free. Reagan, again on the record, stated that unless the American Communist Party was directly influenced by the USSR, the party should not be banned, telling HUAC to let "Democracy do it's thing".
The same could not be said about the ten writers/directors, called "The Hollywood 10", who were cited for Contempt of Congress by a vote of 346 - 17. The most famous of the "10" was the first to testify, John Howard Lawson of the Screen Writer's Guild (Dalton Tumbo was part of the "10" as well). "The Hollywood 10" became heroes to the Liberals and Leftists, and pariahs to Conservatives all at the same time.
Studio Executives, once it became clear that public opinion was solidly behind HUAC, studio execs started to deny jobs to those in "The Hollywood 10", saying that their actions reflected poorly on the film industry. Those executives stated they would not employ any of the "10" until they were acquitted and declared under oath that they were not a communist. The executives also said the policy wasn't just for the "10"; they knew there was a risk, in that innocent people could be hurt, and that creativity suffered when fear was in the atmosphere in Hollywood.
Reagan asked the executives what would happen if HUAC charged an actor with being a communist; the executives responded that if the actor refused to answer whether-or-not they were a communist under oath, the actor would be terminated. Reagan didn't object, since politics were the heart of the matter, far more so than economics. Reagan didn't yet know it, but this was this issue, fighting communism, on which he would build his political career.
In effect, Reagan became an informant for the FBI in Hollywood; Reagan's code-name was "T-10"; Reagan was among at least 18 Hollywood informants, and he never publicized his FBI connections. Reagan didn't think he was doing anything wrong; he judged that he would be far more effective as an informant if he remained anonymous, and the suspected subversives didn't know that he was an informant that ruined their careers . . . as would become clear when he was President, Reagan wanted the atmosphere that surrounded him professionally and personally to be as structured and pleasant as possible . . .