A pure military campaign has some great Rping opportunities, especially if you pit them against ruthless enemies. You could have situations where the enemy indiscriminately kills civilians, commits atrocities and overall act barbarically. Entice feelings of hatred amongst the characters then you can start putting up missions with the enemy in a softer light. POW's taken to be cared for reveal a soft side to the conflict, enemy civilians aren't as bad as they thought, etc...
Getting a real gritty feel to the whole thing is key IMO. The characters should be led first one way then shown the other side of the coin, then they can see the complete picture. Its a great way to develop some in-game stories for characters. And if you continue later on with a more standard exploratory series, the PC can share war stories or meet up with former foes. Works especially well I find with the Bajoran/Cardassian occupation and the Federation involvement.
"The misery of being exploited by capitalists is nothing compared to the misery of not being exploited at all."
-Joan Robinson, economist