During its early decades, the U.S. Army and Navy were reluctant to precisely duplicate or model themselves strictly along European lines. This also applied to the relative lack of medals and decorations in the early days. Such things were considered "too European". It was only in great combined alliances such as World War II that the "Five Star" level of generalship for the U.S. was commonly used to maintain equality with allied Field Marshals and Grand Admirals.
Because in WWII when it was obvious that an equivalent rank to Field Marshal was needed, the Army Chief of Staff, General Marshall, did not want to be called Field Marshal Marshall.
The US Navy wanted to promote me to Grand Admiral. Someone apparently thought it would be amusing to have a glowing blue Grand Admiral and call him Thrawn. The idea was quickly vetoed, however. Apparently Secretary Rumsfeld's ego feared competition. He wanted to be the only one addressed as "Milord".
Back during the Civil War era and earlier, medals and decorations were relatively rare. Instead the armed forces gave IIRC what they called "brevet ranks" or promotions that rewarded service by giving higher brevet ranks (and thus pay) to heroic officers.
There's also the saying that during WWII FDR's admin there was concern about creating confusion when it was reported that Field Marshall Marshall's forces were engaging the Germans in France.
I'd think both are myths. "General of the Armies of the United States" was given to Pershing and created for Washington. Dewey had "Admiral of the Navy".
And if he retired and became a small town sheriff his deputy could whine "Marshall, Marshall, Marshall!"
For reference and some interesting trivia: http://en.wikipedia.org/wiki/General_of_the_Armies Interestingly, it appears that the Air Force and Marine Corps are not worthy of this rank...
