I should have said "invading another country that hasn't attacked you or your allies previously". Taking an invasion back to the original invaders is much more justifiable. My point was that starting a new war has virtually never been a good thing.
For example, if the USA were to attack North Korea to stop the obvious human rights violations happening there, I don't think that would actually be a morally justifiable act. It would be much more likely to lead to more human suffering overall than it actually prevents: this was my intended point.
In the case of North Korea I think we can at least pass judgement retroactively on the Korean War. To me the population sizes of North and South Korea alone make is extremely clear who was right and who was wrong in that conflict.
For example, if the USA were to attack North Korea to stop the obvious human rights violations happening there, I don't think that would actually be a morally justifiable act. It would be much more likely to lead to more human suffering overall than it actually prevents: this was my intended point.