The Military Campaign (CFH)

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Finally, you can run an all-military campaign without it necessarily being an all-Myrmidons campaign.
Possibly the most interesting way to do this is to gradually move the campaign setting toward a major war. Early in the regular campaign, establish that the player characters' country often has problems and frictions with a neighboring country; showcase the disputes between the nations by having NPCs from the other nation be rude or abusive to the PCs (and, to be fair, have NPCs from the player-characters' country be equally vulgar to foreign NPCs who don't deserve such treatment; the PCs may find themselves in the uncomfortable position of being honor-bound to defend the "enemy" from their own dishonorable countrymen!).
As the campaign progresses and the player-characters gain experience levels, increase the friction between the nations. Occasionally, the PCs will run into foreign spy-nests and spy-plans.
Ultimately, when the PCs have achieved high experience levels and are important characters in the nation, they should be in on the events that spark the eventual war. For example, their king may ask them to accompany him to the peace conference where he and his old enemy are supposed to patch up their differences, marry their children to one another, and sign mutual nonaggression pacts. But when they're there, something goes drastically wrong: Perhaps dopplegangers pretending to be the PCs assassinate the foreign king, while dopplegangers pretending to be the greatest heroes of the other nation assassinate the PCs' king. (The dopplegangers, of course, are under the control of the evil king of a third nation, who intends to move in, pick up the pieces, and conquer everyone when these two nations have beaten each other to pieces.)
At this point, war is inevitable, and the PCs are put in command of an entire army. For the remainder of this extended storyline, the PCs have to lead their men in combat against enemy forces, defending their own nation or penetrating into the enemy territory; meanwhile, they'll be doing their own adventures and investigations, trying to come up with proof that the two rulers were assassinated by a third party, not by the heroes of the two nations.
Once the two armies are getting seriously tired and hurt, the PCs should be able to come up with the proof they need. They convince the new rulers of both nations of the identity of their true enemy. At this point, it's time for the two armies to join forces (and swallowing down their many years of mutual distrust, especially after the last several months of fighting, will be particularly hard, leading to even more plot opportunities). Now, the two battered armies march into the third nation, the PCs still in charge of their own army (or perhaps each PC is now a general in his own right), and the final battle with the fresh forces of the evil enemy ruler is still to come...
In such a way, you can sustain an all-military campaign for many months of game-time, and resolve important conflicts and storylines in your campaign world.