I think you can definitely have a full military dictatorship arise from representative government; it's already happening. Once all the pieces are in place (Patriot Act, DHS, NDAA, NDRP, and now drone strikes on U.S. soil against "domestic terrorists"), it just becomes a matter of flipping the switch. Anyone who opposes it, under the NDAA, can be labeled a "domestic terrorist" and executed via drone stike. All the while convincing the masses that the only way to be safe is to have the government/military protect us from the evil within. Half the country is already on board with their push to ban guns saying things like "Only the military and police should have guns". That's how it started in Germany in the 30's.
I'm not a tin foil hat kinda guy, but with all the blatant infringements on our rights (see the list of acts above) over the last 15 years; you've got to be kidding yourself if you aren't concerned.
This very nature of humans means all these calls for security will always result in subjugation and oppression. The ironic part is, as the middle class and poor get wiped out and become less safe, they will call for more action from government to protect them from the predators (white collar criminals and red collar ones) not knowing that these measures only serve to further oppress them.