Corporal-general

A fictitious rank for someone who believes that they are a genius military leader (or at least more competent than their actual military leaders), due specifically to their lower-level military experience.

Typically found among former soldiers who end up in positions of authority, and believe that their being in the army once, and being from the rank and file and not an officer, makes them more qualified to make big decisions than actual officers, despite said officers potentially having made such big decisions for many years, but who were obviously out of touch because they'd started their careers as officers and had spent most of their time behind desks.

Similar to the armchair general, but with the caveat of even the tiniest amount of military service instantly making them more qualified than other armchair generals.

"And so Hitler the Corporal-General, remembering his time as a corporal in the trenches of the Western Front, declared that putting more resources into producing flamethrowers would win the war, much to the chagrin of his generals, who had instead been trying to get him to allocate more divisions to repel the allied invasion of Sicily."