I wonder if a military dictatorship is possible in the US. Since the Continental Army is too weak to beat the state militias, it's impossible for George Washington to become a dictator, even more because he doesn't want it. So I think an interesting era for that would be the Civil War and the Reconstruction era. These times had some characteristic traits making it easier to install an authoritarian regime: strong ideological controversies (slavery and the status of freedmen), social classes oppressed (slaves), social classes threatened by loss of power, a large number of soldiers under arms, strong social conflicts (slavery, reconstruction and the Klan) and plenty of opportunities to limit public liberties for the sake of "security", "order" or "equality".
I don't think that Lincoln could do it in the Civil War itself, but what about Grant? His administration was quite controversial, but had clear goals supported by a large part of the population: punish the south for its "treason", integrate the freedmen in the society and use their votes to secure republican dominance in the South. Now could Reconstruction went so radical that army has to restore order and can enforce a dictatorial order over the country?
Even if a Grant dictatorship would be harmful to democracy, I can see some benefits from it: reconstruction will be succesfull and Blacks' rights will be enforced; democrats in the south will have no chance to regain their position and take away voting rights from freedmen; the US will become much more centralized than OTL, so industrial growth as well as criminal and civil law will be unified; and after the end of the dictatorships, a new constitution might be enacted, so that the constitutional fetishism of the US doesn't happen as OTL. Also, even if this isn't really positive, a military dictatorship could be much more aggressive than the American democracy. I expect that once the "junta" comes under internal pressure, they'll start to look for adventures overseas (American colonies in Africa? Conquests in South America) or even in Canada. The US might become involved in international politics and conflicts earlier than OTL.
Even if I admit that this scenario isn't really plausible, do you think that it's basically possible?