You're moving the goalposts. First you say there is no good reason, then you reframe it 'in the world of realpolitik'. If you want to support your original assertion, you need to argue why realpolitik is the only valid worldview.

> Educating the citizens of a country that you may have to go war with soon is also not a way to further your national interest - if we define US interest as unchallenged US might.

Again, you're moving the goalposts by adding if we define US interest as unchallenged US might and by adding to further your national interest.

Even if you accept those premises, your conclusion would need more arguments. Educating citizens of a country might further the US national interest if they stay in the US after their studies or return to their country and oppose their leadership.