The Ace of Spades. The Viet Cong allegedly feared this card because of the superstitions surrounding it as a harbinger of death.
During the Vietnam war, thousands of these specialty decks were shipped overseas, where troops used them as a form of psychological warfare against enemy forces in Vietnam. Scattering these death cards in real or suspected hostile areas reportedly caused the Viet Cong to flee. In addition, some troops would leave a single ace of spades card on the body of an enemy kill to indicate "We were here" or "We're coming for you."
More recently, decks of most-wanted Iraqis "playing cards" was issued to soldiers during Operation Iraqi Freedom, each card had the picture of a wanted Iraqi official on it.
Saddam Hussein got the nickname "Ace of Spades", as that card bore his image.