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.

In combat, full lives can be snuffed out with no notice paid to the person behind a name. As a veteran of Afghanistan and now a company commander in the National Guard, I am well acquainted with the impermanence of life. Like the American-style graffiti that dominated cities across the country in the 1970s and 1980s, the drawings of war are part of a culture that comes with its own vocabulary, characters and aesthetics. “Graffiti is one of the few tools you have if you have almost nothing,” the street artist Banksy wrote in 2001. “And even if you don’t come up with a picture to cure world poverty, you can make someone smile while they’re having a piss.” If he replaced “cure world poverty” with “win the war,” he would have perfectly captured the sentiment behind soldiers’ doodles. These drawings, scratchings and markings serve a far greater purpose than merely offering a glimpse into the past: They are a defiant and public proclamation of a human being’s existence.

[Send us photos of your favorite graffiti]

During the Civil War, American troops from both sides scratched images and names onto surfaces all over the war-ravaged landscape. In houses, churches and public buildings, scribbles and drawings express sentiments ranging from common statements of what unit was there and what they did — “The 91st Ohio got dinner on the opposite [side] of the creek July 19, 1864” — to creative insults directed toward the opposing side — a curse placed on the Confederate president, Jefferson Davis, calling for him to be eaten by a shark that will be eaten by a whale, that will then go to hell — to modest pleas for kindness: “[Who]ever shall read this wall, please remember me in prayers.” Since many towns in Northern Virginia changed hands multiple times, soldiers often had the chance to respond to the graffiti left on walls by their adversaries. These early exchanges of “trash talk” are still preserved in some historic homes.