About 5,000 years ago, someone decided to paint a battle scene between archers in a cave in Spain — perhaps one of the first instances of what we’d call “war graffiti” today. That person was probably an early grunt who had just finished griping that the chow was bad and that he’d had to march too far that day. Because as long as there has been war, there have been soldiers leaving behind their doodles, names or other markings for historians to muse on why they did so.