The letter appeared on the scene in 1955 in a book by William Guy Carr (June 2, 1895 - October 2, 1959) named Pawns in the Game (1955). Hoax or not a hoax, the third part of the World War plan was first sited in a 1894 book Le diable au XIXe siècle
From here
http://en.wikipedia.org/wiki/William_Guy_Carr
"Carr states that he learned of this conspiracy from a letter written by Pike and addressed to Italian revolutionary leader Giuseppe Mazzini."
"Carr states that he learned about the letter from the anti-Mason, Cardinal Caro y Rodriguez of Santiago, Chile, author of The Mystery of Freemasonry Unveiled"
"The reference Terry Melanson gives a good treatment to the three world war theory. Michael Haupt had taken the three world war theory from the introduction of Carr's Pawns in the Game(1958). This introduction outlines a plan that Carr attributes to Pike, but not to the letter from Pike to Mazzini. Only the last section of the three world war plan in Haupt's text is a quote attributed to the letter from Pike to Mazzini. This quote is virtually identical to the one in Rodrique's book and it can be traced to the book Le diable au XIXe siècle (1894) by Gabriel Jagond-Pager aka Leo Taxil, where it is claimed to be from a letter of Pike to Mazzini written in 1871. This quote was later considered to describe the Bolshevik revolution, but whether a hoax or not, it predates 1917."