You're the one trotting out a simplistic black and white vision as if anything about any part of history is or can ever be explained in such terms. History is always much more complicated than our ideological biases would like.
There's zero context in your comment. It's just as biased as the meme is. You're blithely glossing over the much larger historical context of WW2 and why the US was there in the first place, and you're eliding the rather obvious fact that a sizable majority of Koreans were opposed to the attempted communist takeover in the first place.
The salient fact about the 2nd half of the 20th century, that is routinely ignored by Lemmy's tankies, is that the men guiding US foreign policy had survived the largest war in human history and were absolutely and legitimately terrified that there would be another even worse war in the very near future if they didn't do everything they could to prevent the kind of runaway imperialism seen in Nazi Germany and Imperial Japan.
Furthermore, these men knew for a fact --as even Lemmy's tankies will admit-- that communism by design and by doctrine can only come to its final stage in a globally hegemonic system. If you honestly believed, as they clearly did, that fighting a war in Korea --which after all had been liberated from Imperial Japan by the US-- was part of a much larger strategy to contain communism and thereby prevent a 3rd world war, you would feel yourself morally obliged to do it.
We can argue about whether or not they were correct in their beliefs, but we can't simply condemn them as evil imperialists. That's just stupid reductionist bullshit. Reality is always much more complicated than simple black and white "my team good, your team bad."