Recollections of Ronald Reagan’s time in Hollywood are full of myth and misinformation. From the right’s perspective, his role as a union man who campaigned for Truman is conveniently forgotten; the left, on the other hand, has held tight to its view of him as an archconservative Red baiter.

What is indisputable is that the 40th president of the United States came to office with a deep-seated hatred of Communism. But less known than his commitment to its end was that he already had a plan for seeing to it. In fact, it was during the bizarre McCarthy-era Red Scare that the seeds of his campaign against the ideology were planted.

The key to unlocking and understanding Reagan’s evolution was found in 75 legal-file boxes in a house...