After a 130 thousand years of all human beings living in the south of Africa, some few were pressured by food shortage to head north into Europe. Far from the equatorial sun, the palest of them survived best because only their skin let in enough ultraviolet to make Vitamin D. After a long time of not much, they found that their land held the largest deposits of iron on the planet. This iron made them very powerful. It made for plows, swords, armor, and later, guns. That happened in the past 10 thousand years.Only a few hundred years ago, the pale ones “discovered” the continent they had left ages ago. In Europe they were used to fighting each other for land, metal, money, and workers, and here they found these goods undefended by swords, guns, or horsemen. The people they found had very different markers of culture, such that each seemed uncultured to the other. The pale ones wanted to steal the land, metal, money, and workers and not much was stopping them beyond morality. They got past the morality by using their own pale skin as a marker for quality, which they backed up by lamenting the absence of salad forks on the African continent, and then they stole everything they wanted. They treated the human beings there as animals.
For a quick response written in the small hours, that's not too bad a synopsis of Jared Diamond's Guns, Germs and Steel, come to think of it.