South Korea was a desperately poor nation when it was invaded by the North Koreans, and then the Chinese, in the 1950s. American troops were sent, and succeeded in creating a stalemate. They are still there today. In the meantime, South Korea has evolved from a politically repressive basket case into a genuinely democratic state with a first world economy.
American troops also succeeded initially in creating a stalemate in Vietnam. After the troops were withdrawn, however, it became clear that the South Vietnamese government and military lacked the willingness and the ability to hold off the enemy on their own. The result was a defeat and a blow to American prestige, although, in the long run, it probably didn’t matter much; today, Vietnam is more of an ally to us than to its erstwhile patrons.
Why were the outcomes so different? Two factors jump out at you. First, the American military commitment to South Korea was open-ended, while the commitment to South Vietnam was not. Second, and more important, the South Korean government was a success story; the South Vietnamese government was a failure. The ultimate success of the mission thus depended largely on factors that were mostly out of our control.
Which of these models more closely resembles Afghanistan? You decide.