People from north Yugoslavia in 1991 probably thought it would be pointless for the Yugoslavian government to attack them. Actually majority of all Yugoslavians probably thought so. Algerians probably thought so too and Libyans and Egyptians.

If you wish to look at democracy that fell to junta, check out Burma. And here is story about failed junta. It failed because of widespread civil disobedience.

Now if you think about the year 2015 or 2030 I think you are completely right. But your constitution has stood for more than 200 years. What will U.S. look in the year 2213? What kind of laws you should now have to ensure a nice democratic state? You don't have marines vs. army and air force vs. navy just because it's handy to separate these things. You have your force divided to four sectors so that it would be difficult for military junta to form simultaneously at every military sector.

Here in Finland the setting is completely different. Government doesn't have that bad monopoly of violence as it relies on civilian reserves for defense. And we don't have two party system.

PS. Then there is bad stuff that doesn't require an actual attack. How would you feel if government would issue obligatory surveillance cameras to every household?