Hummmm, I think its possible not for her to cheat. So much digital communication, so I still have hope. Not everyone cheats.
How long will the person be deployed? If you can visit every 4 months then IMO its doable. Also,
I would leave it at a "Open" relationship. I had a woman propose this to me, because she was a flight attendant, so it made sense since she was constantly away. Looking back I should of just agree with her but I got instantly attached, she was so hot. scale of 15, not 10. With a Psychology Degree.
Here's something I recently learn, some may not agree, but - She's human too. Would be it so hard to accept the reality and know she could end up having sex with some one else? Can you be man enough to get passed it? I think and probably can. Think about Porn stars or strippers, would it be any different? Some are married. I dated a stripper and not embarrassed to admit it. Took some emotional strength, but it can be done.
If a person is leaving for a "long" term, you seriously cannot be selfish and expect someone to wait on you? Unless you propose and make every effort to stay in touch. IMO It is possible. Gentleman, if you find yourself in this position, then learn the ropes, don't demonized the woman.