I'm no Trump supporter, but I think he's not advocating for colonialism at all. He means that if the US is fighting a regime or organization which poses a threat to America, the military should pursue unconditional surrender and not half-measures, like imposing artificially democratic regimes just to invade again 5 years later. He is right about this. Which is better: post WWII Japan, or Iraq, with its amazing democratic government and ISIS?
Besides, the whole concept of war crime is ridiculous. Every war fought by the US in the last 70 years has been a crime in and of itself.