The history of the Ace of Spades representing death dates back to ancient times. The symbol marked the beginning of winter - which often lead to famine and even death for common people. The practice of placing these cards on the bodies of dead enemies began in Vietnam as an effort of psychological warfare. The Death Card targeted the superstitions of the North Vietnamese - the Ace of Spades became the symbol of death, humiliation, and suffering.
Soldiers wore the cards on their helmets and took photos with them - the cards were distributed after battle, often left on the fallen enemy's eyes. The symbol was also used on leaflets paired with a skull warning the North Vietnamese troops to cease combat, or face death.
After time it became clear that the symbol became more of a morale booster for the American troops.