In the latter case it is often government that organizes the conquest, and religion that justifies it.
Jared DiamondMuch of human history has consisted of unequal conflicts between the haves and the have-nots.
Jared DiamondEurasia ended up with the most domesticated animal species in part because it's the world's largest land mass and offered the most wild species to begin with.
Jared DiamondTwelve thousand years ago, everybody on earth was a hunter-gatherer; now almost all of us are farmers or else are fed by farmers. The spread of farming from those few sites of origin usually did not occur as a result of the hunter-gatherers' elsewhere adopting farming; hunter-gatherers tend to be conservative.... Instead, farming spread mainly through farmers' outbreeding hunters, developing more potent technology, and then killing the hunters or driving them off of all lands suitable for agriculture.
Jared Diamond