If you wish to learn how it came to pass via historical revisionism, read "In Denial: Historians, Communism and Espionage" by John Earl Haynes and Harey Klehr.
These truths are disheartening to me after 40-years of being scoffed at and called paranoid by friends and relatives when I tried to make them see what was happening. The greatest victories won by the "progressives" were their takeover of academia and the media (including "popular culture.")
Don't I know. It's sometimes like one of those old low budget horror films, where one person can see the monster and everyone around him is blissfully unaware.