Although the word "ball" might bring to mind the princes and princesses from the fairy tales of yesteryear, a military ball is a significant event that shouldn’t be taken lightly. In the military, elaborate balls pay tribute to those who serve in every branch of the armed forces. But don’t let those images of formal finery that initially invaded your mind disappear, because a military ball is a formal event for both the enlisted and the civilians in attendance.
I would say something very formal,other people have said things like prom dresses
which is true,but I wouldn't do a really poofy one and try to be a little more conservative as in, more, pretty/elegant, then hott or sexy, more like a fitted one,probably in black or some color that's not totally bold.
I'll add some links for some dress styles that would most likely be fit for the occasion
Some Formal Dress For Military Ball:
|Pink V-Neck Floor Length Dress|
|Tiffany Blue V-Neck Floor Length Dress|
|Red One Shoulder Floor Length Dress|
|Purple One Shoulder Floor Length Dress|
|Aqua Cloak Floor Length Dress|
|Teal V-Neck Floor Length Dress|