The, perhaps naive, impression of someone who had history classes in high school is that the human society in the past put a lot of effort into expanding borders.
Empires and kingdoms rose and fell and seemed to be often, even constantly, involved in an effort to extend their borders.
What was the main driver of this expansionist drive?
Was it specifically to do with scarcity of resources? This would seem a very natural response, except that this behavior is often seen in times and places where (arguably) the costs and difficulties involved in the wars might offset the gains.
I realize that this is generic, and different contexts might have different drivers, feel free to give examples!