From reading several sources, the best I can tell is the following. On the Bicentennial, July 4, 1776 George Washington was postumously given the rank of "General of the Armies of the United States" with the further distinction that he was also said to be the "Highest rank past or present." Only one other General was given the title "General of the Armies of the United States" (notice the plural armies) and that was "Blackjack Pershing." At his point in history, however, that rank was associated with four stars. The "five-star" designation occurred during WWII and the title "General of the Army of the United States" (note: singular "army") was associated with it.
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