Laos and Cambodia weren't participants in the war but that doesn't really feel like it matters all that much. Whenever I talk about the bombings of Cambodia and Laos with Americans (who - liberals and conservatives alike feel they must always defend) I sometimes here "well we bombed cities in Germany and Japan in WW2 and no one talks about those being war crimes". But were they? I really don't know much about those bombings. My gut says yes they were also war crimes but we just accept them because they were combatant countries?
I think on the surface it would be justified to kill in order to stop those genocides.
But there is apparently evidence that the whole allied bombing campaign over Nazi Germany targeting civilians was actually just prolonging the war.
Because Germany was not at all democratic, there was not any real pressure applied by bombing civilians. Additionally, the main victims were city dwellers, who were more likely to be opposed to Hitler, however quiet that opposition was. So we kinda just killed his German enemies for him, which he wouldn't have been able to do without some real work to justify it
I'm not sure how it applied to the bombing of Japan but it could be a similar dynamic.
Turns out that murdering entire cities of defenseless civilians really motivates people to fight harder, especially if they're convinced that you'll exterminate them if they stop fighting. British command didn't care though, they had power over people and they had the moral and emotional intelligence of a kid lighting off fire crackers in an ant hill because one of them stung his toe.