alt Hacker NewsThe South Korean government was a military dictatorship at the time. And the North was wealthier. Without the power of foresight, you couldn't justify the war as bringing democracy or economic prosperity, it was purely a geopolitical struggle. It took a lot of historical revisionism to erase that inconvenient little fact from western consciousness.
The majority opinion at the time, in the west, was that communism led to less economic prosperity, less democratic institutions, and less freedom. So maybe it wasn't a guarantee that the south would become a democracy or prosperous, but doing nothing would guarantee the opposite.
Also, the North wasn't democratic either so it was two nondemocratic groups fighting.