Hey Habu,
I'm also no expert, but my field of interest is also US history (although not military history), and I am somewhat familiar with the programs at the two schools you mentioned. I think your GRE scores and cumulative GPA are good enough to merit consideration at Duke and UNC. A lot will depend on how good your SOP and writing sample are.
It's impossible for anyone to say with any accuracy what your chances are. Those two programs are ultra competitive ( I think Duke only matriculates maybe 10 per year). Some of it also depends on things completely out of your control- will the professors that are interested in your application be able to take new students, and how much money is available for funding this year? For the record, it sounds like your interests are by far a better fit for UNC than Duke. I remember when I researched Duke, almost all of their military history/civil war scholars were either retired or close to it.
Are those the only programs you applied to, or just your top two choices? Best of Luck!