Countries have disputes and go to wars with their neighbors, and neighboring areas are historically mixed populations, which drives tensions as to who owns what, and such, often resolved in wars (which are seldom one sided: historically the offending sides alternate). And usually those countries are of the same or similar development stage, plus they have to live close together, as populations, so enslavement is rarely if ever the case, nor is it made about racial inferiority.
So, yeah, "the neighboring warlord who arriving on a horse" is not some 10-100x stronger and richer global power coming to the other side of the world to make your people slaves, considers you racially inferior, displays you in human zoos, and even declares open season and hunts you like animals, while they steal your resources...
So there's that.
This is just demonstrably untrue. Plenty of neighboring warfare is still waged on the basis of race. Look at the history of the Middle East. The history of Europe itself. Look at Israel and Palestine. Look at Japan and Korea's history of warfare... or really Japan vs any of its neighboring Asian countries. Han nationalism, if we want a Chinese example since that's what this topic started with. All of these examples also involve slavery at various levels.
Being neighbors does not in any way shape or form make for "better" or "gentler" war as far I can tell from history. Your point just doesn't hold up to obvious historical data.
Bear in mind, I don't say any of this to absolve the evils of colonial powers. It's just strange to me that in the modern day it's treated so differently when it doesn't really look like a deviation at all from man's "usual" cruelty.
E.g. occupying a position of a "decolonization" officer somewhere (some institution of higher learning?) and having a secure job with no performance reviews + power over others until they retire.
