My theory why Frank Merrill was a brigadier general
In the United States Army, a formation of 3,000 soldiers is called a, brigade and it is commanded by a full colonel. The word, 'full', is not official but colloquial, to distinguish it from, lieutenant colonel. In informal conversations, both ranks are often referred to as, 'colonel'.
In the history of the U.S. Army, brigadier generals often followed European military tradition from the late 16th to 19th century by being the final, highest ranking fighting officer rank. Major generals and higher did not personally lead men into battle but brigadier generals could still do so and did so, which is why the term was adopted from the French, 'generale de brigade'. In military history, whenever a brigadier general commanded a brigade, it was typically 5,000 men. In the U.S. this practice continued into the Civil War where a brigadier general could command a unit as low as 5,000 to a full division. The Union cavalry division of the 9th at Gettysburg was a brigadier general and his division was only 9,000 men compared to the authorized strength of 16,000 of Union and Confederate armies.
Thanks for reading all the above background info and I apologize if I bored you.
I was surprised that a brigadier general was in charge of only a brigade sized unit, the 5307th Composite Unit (Provisional). The mid to late 20th century American Army rarely uses brigadier generals except for two armored cavalry regiments.
Here's my theory. It was political. The Americans needed to demonstrate to the British that they were fully committed to the China-Burma-India theater. The truth known to all was that the U.S. couldn't spare many more men after Europe and the Central Pacific. But it still had to be done. Placing an officer of general rank in charge of a large, commando-like formation of 3,000 men would surely impress the British and show them the Americans were serious. It worked but as the movie shows, the unfortunate brigadier general Merrill had to literally work his men to death.