By the time the United States entered World War I, the conflict had been raging in Europe for nearly three years. It was to become one of the deadliest wars in human history, claiming more than 15 million lives. Advances in military technology—including more lethal artillery and rapid-fire machine guns— contributed to the heavy toll. Maps, too, played a role. Recent cartographic innovations allowed artillery gunners to fire at targets they couldn’t directly see and aim their guns without first firing “ranging shots” that would ruin the element of surprise.