"This is a Sign of Death!"

"Viet Cong! This is a sign of death!
Continue your struggle against the National Cause
and you will surely die a mournful death like this!"

Using a variation of Pavlov's dog experiment, Ace of Spades leaflets were dropped over an area and an hour or so later the area was heavily bombed or shelled with artillery. The North Viet Cong quickly learned to leave the area when they saw the ace of spades because that meant a bombing/shelling of the area was imminent. Once "trained to react" the leaflets could be dropped into an enemy held area, the enemy would withdraw, and the US Forces could move in without the shelling or bombing.