Michael Delong said this quote

Every war and conflict that the United States enters has its own ROE [rules of engagement]. Contrary to what most people think, the U.S. military does not have a complete license to kill, even in wartime. We are not a barbaric state, and we do not enter any war with the intention of unilaterally killing anything in our path. We go out of our way to spare civilian lives, to keep those who are not in the war out of it--sometimes even at the expense of risking our own soldiers' safety. We do this by creating strict rules to which our soldies adhere. These rules govern when they can fire, when they cannot; what type of force they can use, what type they cannot; what they can do in particular situations, and what they cannot. The reason for this is that battles can become very confusing very quickly, and a common soldier needs simple rules to guide him, to know when he is or is not allowed to kill--and who is and is not the enemy.