Those wars in Cambodia and Vietnam didn't further the interests of the US at all. They just wasted tons of lives for nothing. Same as with the recent Afghanistan campaign.
At least the military industrial complex got even richer of it. That's the only reason.
Between 1965 and 1975, the United States and its allies dropped more than 7.5 million tons of bombs on Vietnam, Laos, and Cambodia—double the amount dropped on Europe and Asia during World War II.
Pound for pound, it remains the largest aerial bombardment in human history.
Japan's "amazing recovery" wasn't hampered by a legacy of UXB (unexploded bombs) that still kill and cripple children to this day.
America had total control of Japan following their surrender, and the time/power/resources to rebuild Japan as they saw fit (which was to become an eastern bulwark of capitalist freedom, against China and Russia).
Bombing Cambodia had the much more cynical purpose of convincing Ho Chi Min that Nixon was an unrestrained madman whose demands in peace talks had to be surrendered to, to avoid further mindless devastation for all involved. Yes, it was more complicated in the details, but pretty damn clear in the larger picture.
At least the military industrial complex got even richer of it. That's the only reason.