Defense
Defense or combat refers to the act of simply defending oneself or going a few steps further and attempting to violently conquer other lands to make them your own. Eventually, when you're expanding your settlement, you're going to come to enjoy all the progress and resources you've made, and sadly, so will others who want to take it from you. These are known as raids, and without a skilled army to fend them off, you will fall victim to these raids.