That was the harvest. The sowing was at least back in the 30s: The Frankfurt School and The Long March Through the Institutions.
They succeeded in the 60s because Marxists had already infiltrated and hired and promoted their own.
That is the key. A self-conscious, organized and determined minority can wield vastly disproportionate influence in large institutions. If that minority hacks out the ability to control hiring and promotion, it is a threat take over. Once that is done, it will act as a gatekeeper to perpetuate its control. What the left has done in the universities is similar to what the gay mafia has done in the Catholic hierarchy.
True. If you want to do deep history you can look back to the founding of the Fabian Society in 1884, whose goal was to spread so-called "Democratic Socialism throughout the world. Members like George Bernard Shaw and H.G. Wells lent it a certain intellectual chic that was happily propagated by American colleges and universities. I remember getting a big dose of it myself as a college student in the '50s.