Technological magnification
Until the twentieth century most military action was performed by individuals in relation to other individuals. However, with the development of automatic rifles, long distance tanks, bombing from the air and underwater torpedoes, this had the effect of depersonalising the enemy and making him (or her) invisible.
This comment is not entirely fair, since the practice of siege warfare goes back to the Persian Empire of Alexander the Great. However, the changing scale of technological warfare, isn’t simply that of bows, arrows and swords to the world of gunpowder, it’s magnified to an entirely new degree (and how much more so in the century since).