War put an end to the governmental systems that were specifically based on these ideologies. The underlying ideologies of Fascism and Soviet Communism still thrive in American liberal fever swamps. If you don't believe me, spend a week over at DU or the DailyKos. Of course, you won't come back the same person.
You are running along he right track; just look up for the bigger picture.
Once you admit that there are bad governments out there that need to be contained (the Soviet Union immediately after its formation) or destroyed (Germany 1940) you have destroyed the bedrock of unrestrained liberalism - “everything is relative”. A hard value judgment with a black or white answer is a threat to liberalism.
Besides, once a student makes his first value judgment on something, and is proven right (had his judgment confirmed by others0 what is to prevent him from doing so again? And again? And so on until his value judgments start affecting the professor’s life style.
A study of WAR forces the student to make a yes/no value judgment. And, once he learns that skill, the entire liberal establishment has lost his support (vote) and money (vote).