Consider it an economic problem. War costs money, therefore war will be conducted when its benefits exceed its costs. What makes war profitable? For that you need to know the benefits of war. The most obvious benefit is the changing of borders and conquering of new land. Now you might say that it always makes sense for a country to try to conquer the enire world, because having the land grants you access to its riches for free, but then you have to consider the costs of war too.

The cost of conquering land and managing it yourself must be cheaper than letting the other country manage the land and production process and then importing the goods you want.

The cost of conquering the land can be split into two parts. First, the initial offensive representing capex and the ongoing defense costs against attackers represent opex. If it turns out that the foreign country is both fleecing you on imports and is spending less in defense per acre than you would, if you managed the land, it would be profitable for you to conquer the land and install a defense force that spends more to protect the same plot of land. Since your spending budget is higher per acre, the original country would lose if they tried to recover the territory.

Now consider the opposite scenario. The original country does not fleece you on imports and has a lean army. Then it would not be profitable for you to conquer the land, since you have higher opex and the benefit is insignificant.

Of course none of this stops crazy dictators from attempting world domination, but it can tell you that on average, people should be uninterested in war.