score:30
This is a famous if apocryphal letter, traditionally attributed to Wellington during the Peninsular War, though I wouldn't be surprised if a variant floating around transposed it to America a generation earlier. It is addressed to the War Office not to the King directly (which would in any case have been very anachronistic for the period) but otherwise seems very likely to be the one you remember.
The bit below is quoted from this version, but searching on key phrases will bring up many others, all more or less the same:
This brings me to my present purpose, which is to request elucidation of my instructions from His Majesty's Government so that I may better understand why I am dragging an army over these barren plains. I construe that perforce it must be one of two alternative duties, as given below. I shall pursue either one with the best of my ability, but I cannot do both:
To train an army of uniformed British clerks in Spain for the benefit of the accountants and copy-boys in London or perchance,
To see to it that the forces of Napoleon are driven out of Spain.
However, it is almost certainly apocryphal. Poking around on Google Books did not find any hits for key phrases ("train an army of uniformed British clerks", "dragging an army over these barren plains", etc) before about the 1980s. Most of the times that it appears in more formal publications (eg here) it is cautiously labelled "attributed". No such letter appears to exist in the edited Dispatches of Wellington from the Peninsula.