What's the highest rank available?
In the US Army, Air Force and Navy, the highest rank is an O-11 - General of the Army or Air Force and Fleet Admiral. Although there were men who had this rank prior to WWII (Grant, Sherman, and Sheridan) there was no 5 star rank. The first time the 5 star rank was worn was during WWII.
The reciepients were:
• Adm William D. Leahy December 15, 1944
• Gen George C. Marshall 16 December 1944
• Adm Ernest King December 17, 1944
• Gen Douglas MacArthur 18 December 1944
• Adm Chester Nimitz December 19, 1944
• Gen Dwight D. Eisenhower 20 December 1944
• Gen Henry H. Arnold 21 December 1944
• Adm William Halsey December 11, 1945
• Gen Omar Bradley 20 September 1950
No doubt their date of rank was carefully planned out. These men held that rank until retirement so you had big Mac wearing his in Korea. Marshall retired in 1945. Eisenhower was chief of staff of the Army until 48 then became Supreme Commander of the North Atlantic Treaty Organization before becoming president. Arnold had four heart attacks during the war requiring hospitalization so he retired right after the war. Bradley was not actually promoted during WWII but afterward when he became Chairman of the Joint Chiefs of Staff.
Of course those in power at the time were wise enough to not grant this to any Marine general or who knows what would have happened!