Not to mention that much of the leftism in US academia started with the Frankfurt school and some other degenerates getting yeeted out of Germany, as well as scum like
Jean-Paul Sartre - Wikipedia, and the
fabian scum and other morons.
No, the leftist conquest of academia came long before than in the US. You see, in the US the majority of elite colleges were actually religious institutions founded by various Christian Churches.
In the late 19th and early 20th century you saw the rise of a group of "intellectuals" in the Protestant Churches known as the "Modernists". These individuals wanted to "Modernize" Church teachings and bring it into accord with what at the time was modern science, archeology, and history thought was true. Suffice to say, since all those were completely "materialist" and they arbitrarily ruled that the Bible didn't count as an ancient text, they'd "discovered" that all the Bible was mythical, that Jesus wasn't a historic person*, and that those who held to the Bible being literally true were unscientific and didn't deserve to be part of the academy. Thus they staged a systemic purge of the academies and leadership of mainline Protestant academia and senior Church leadership of those who did not believe as them.
What this ended up doing was purging actual religion from the American academy and replaced it with materialism and philosophy. When Marxism made it's way to the US, it found fertile ground in the academy since, well, Marxism appeals to those who have no religion while promising heaven on earth.
All this happened in the 1920s and 1930s. The reason this took so long to have an impact is that prior to the 1960s college was really only for people going for certain specific elite jobs like medical doctors or high level science where you needed that extensive intellectual training. Even lawyers in the US were as much a trade as an academic field, with many states not requiring college degrees to sit the bar exam, being able to instead learn the law via apprenticeship. It wasn't until the 1960s and the Baby Boom generation that you began to see college as a more normal and expected thing for people to do as part of their career path, and, obviously, it is with that same generation you begin to see things go wrong. At the same time in the 1940s and 50s you did see that communist infiltration get underway.
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* Note, this is a position that is no longer taken seriously by secular scholars, the vast majority now agree that Jesus was a historical person in the 1st Century AD, existed and taught, and that those teachings were then used to found what became historical Christianity. Further, many parts of the Bible have since been verified by further archeological findings, including many parts of the Old Testament. Before you buy into that these late 19th and early 20th century "scholars" were right, remember that these were the same people who thought phrenology was an accurate measure of people's intelligence and capability and basically assumed all surviving ancient stories were mythical in nature, IE, they were the ones who decided Troy didn't exist. Pretty much all archeological assumptions of the period have since been overturned, and most modern scholars are in more agreement with scholars from BEFORE this period about ancient times and the historicity of things (pre this period people accepted those ancient texts, including the Bible, were at least broad strokes accurate, if perhaps embellished for drama) than that are in agreement with these scholars. These same scholars are also those who just made up vast swaths of historical theory like the pan-European witch cult hypothesis which has since been thoroughly discredited, and also made up Wicca pretty much whole cloth as a "recreation" of pre-Christian Celtic religion and, well, it has absolutely nothing in common with those traditions. Basically, scholars of this period literally attempted to rewrite history by rejecting the preserved traditions because they believed themselves to be more "Scientific" than previous people and basically got everything wrong because of it... It would be hilarious in hindsight if they hadn't done so much social damage...