Hi Yeppie,
Actually, No Matter what we do or say, the final decision lies with you. We can give you tons of advise but whether to heed it or not, its still your choice. I think she's might also be seeking friends advise just like you are doing now. 'Not being able to understand each other' may be due to miscommunication or lack of time for communication.
Instead of sittting there and thinking of the worse, blaming yourself or anything else for it, take step back and view the problem from another angle, Its not easy I know, But it really helps.
Sulking and not talking about it don't solve the problems at all, neither will not communicating after the incident help.
You did not elaborate on what exactly happened thus I can only help base on my instincts. You have your own problems, in this case, its the Army, and hers might be her stressful academic or working life. The army is one of the popular reasons that couples breakup. If he both of you really cherish each other, try talking the probelms out, Including Hidden feelings that you think will do no help even if expressed coz you might be surprised that it might be these little hidden words that solves the entire problem.
The rule of the thumb is BE positive, and try not to dwell on whatever that has happened and stop there waiting for each other to take action. Do not use 'YOU Should...', "I Never..." during the discussion as you would put the other party on a defence, instead try using "WE should...".
Good luck...