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FWIW the US had 25,000 - 30,000 troops in Iran from late-1943 through early-45. The core of the logistics, engineering, and railroad units, were specialized troops, but the majority of the troops were non-specialized troops drawn from infantry and motor transport units.
There is a pretty good history of the USMIM (U.S. Military Iranian Mission), IISC (Iran-Iraq Service Command), and PGSC (Persian Gulf Service Command), in the official History of the United States Army series, but I do not think it is available online. A brief description of the British involvement followed by a summary of the US involvement can be found here:
"
US Army TS Transportation 3: Chapter 9: The Persian Corridor"
Note that the above description is a relative clean summary and does not cover (for the most part) the worst behavior of the Allies in Iran and Iraq.
As an interesting side note, for many years after the war and through the 1990s (at least) there was a definite attempt to keep knowledge of the history of the WWII US involvement in Iran and Iraq, as well as knowledge of the US involvement after the war, from the citizenry of the US. Presumably because those in power felt it would not play well to the US citizenry when the image the US government and US oil industry were trying to sell to the public was of a benevolent US facing dastardly and ungrateful Iranian and Iraqi regimes/peoples who could not possibly have any reason to dislike the US.
In a 2003 survey, it was found that less than 0.2% of the US adult population, and only about 1.5% of the population who were adults during WWII, were aware that the US had been involved in the invasion of Iran and Iraq during WWII.