In the Vietnam War, the term “Willy Peter” was used to refer to white phosphorous. The term was used simply because the initials are the same as the initials of the substance. It was also sometimes called “Willie Pete.”
White phosphorous was used in Vietnam mainly in artillery rounds and mortar shells. It could be used for marking positions or for making smoke screens since it creates large clouds of white smoke. It was also used as an incendiary substance, which was much more controversial. White phosphorous could get on the exposed skin and burn the victim very badly as it cannot be permanently extinguished by water.