You are prtty much where you need to be. You have a general area of interest, as David says, but you need to narrow it down. That is something for you to work out with your committee - chair and 2 members at least one of whom should have a PhD. Among the people you might consider talking to and as potential committee members or chairs are: Jim Willbanks and Chris Gabel in Mil Hist, Geoff Babb in DJMNO (or whatever we call it now), Geoff Demarest and Les Grau at FMSO, Rob Thornton at JCISFA (and of course this board), and, of course, Bob Baumann Dir, Grad Degree Programs. Talk with all of them about what you have in mind and ask for advice on committee members.

Good luck

JohnT