Lessons of war

Reading The History of the Ancient World by Susan Wise Bauer has been great. It is not exactly a "page turner/stay up all night until you finish it" book, but I am loving it. I have been trying to figure out why the power of a king was measured by his kingdom's borders--well, besides the obvious reason. Why was it (and is it) so important to be expanding your influence? For example, Sparta had little access to a navigable river, so trade was difficult, and space limited. Therefore, they conquered a neighboring valley with lots of fertile land and subjected the inhabitants to slavery. So they could have food. WHAT? What about figuring out what they did have (besides swords) and trading it for food. The Phrygians (what a cool name) figured out the whole trading thing, but missed the whole defense lesson somewhere along the line and were first subject to the Assyrians, and then wiped out by barbarians. Hmmm. How do we find balance? What guides our actions? Am I patient enough to discover the best way to deal with each situation I find myself in? Or do I jump in, motivated by pride or "honor" to do something?