My theory
(From Wikipedia- Ace of Spades)
War
The ace of spades has been employed, on numerous occasions, in the theater of war. In the Second World War, the soldiers of the 506th Parachute Infantry Regiment of the American 101st Airborne Division were marked with the spades symbol painted on the sides of their helmets. In this capacity, it was used to represent good luck, due to its fortunate connotations in card playing. All four card suits were used for ease of identification of regiments within the airborne division following the confusion of a large scale combat airborne operation. Battalions within the regiments were denoted with tic marks or dots, marked from top clockwise; Headquarters at the twelve o'clock position, 1st Battalion at the three o'clock, etc.
Some twenty years later, the ace of spades was again used by American soldiers—this time as a psychological weapon in the Vietnam War. US troops believed that Vietnamese traditions held the symbolism of the spade to mean death and ill-fortune and in a bid to scare away NLF (Viet Cong) soldiers without a firefight, it was common practice to leave an ace of spades on the bodies of killed Vietnamese and even to litter the forested grounds and fields with the card. This custom was believed to be so effective, that the United States Playing Card Company was asked by Charlie Company, 2nd Battalion 35th Infantry Regiment, 25th Infantry Division, to supply crates of that single card in bulk. The crates were often marked with "Bicycle Secret Weapon".[8] (The Bicycle was the style of card reverse pattern).
The ace of spades, while not a symbol of superstitious fear to the NLF (VC), did help the morale of American soldiers. It was not unheard of for US soldiers and Marines to stick this card in their helmet band as a sort of anti-peace sign.
More recently, in 2003 a deck of most-wanted Iraqi playing cards issued to US 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 his was the face which adorned that card.
http://en.wikipedia.org/wiki/Ace_of_spades