tin-pot (also tinpot, tin pot), adj.
Typically found in the context of generals or dictators, in which case it is invariably derogatory, denoting someone whose capabilities are inferior to his rank. This is the basic meaning but in practice it's more often used in a broader sense. As in, one who imagines he has more power than his position carries, or harbours delusions of grandeur beyond his means and capabilities. Tin is regarded as a lesser material for some metal objects--most notably pots--to be made of, though I also get the mental image of children playing soldiers wearing saucepans on their heads for helmets.
The tin-pot general is the type of officer who is liable to engage in ill-advised manoeuvres for the sake of commanding an operation, or to harass civilians because he can, and is closely related to the stereotypical portrait of a small, blustering man in uniform who suffers from an inferiority complex and who acts like a child bossing around his playmates.
A tin-pot dictator would be the sort who imagines himself as being an international statesman and/or military genius, and conducts himself in a manner inconsistent with his actual (diminished) prestige outside his small realm of absolute power. These are the ones who are likely to embroil their country in military escapades outside their borders. Idi Amin might be a good example.
Also, a pot made of tin.
The word first appears in the OED as early as 1838 but is not found in less comprehensive works.