LOW LEVEL CAMPAIGN IDEAS
At low levels, the characters fit most obviously in as regular soldiers. Even players with non-warrior type heroes can be easily fitted in - a thief may have been offered military service instead of imprisonment, a priest may be part of the army for religious reasons, and a wizard's knowledge of powerful magics would surely be highly prized by the military.
In addition, the characters can be easily moved along by the story in a credible way - they may be detached from their unit, given special orders or captured by the enemy and forced to escape.
If it is to be a long running campaign, then your players will have the opportunity to climb the rank ladders and possibly lead units or eventually armies. Portraying the armies commanders as incompetent from the start of the campaign will make this achievement sweeter.
HIGH LEVEL CAMPAIGN IDEAS
At high levels, characters will have a far greater influence over the course of the war, they may even be army commanders or members of the civilian government. Military campaigns work particularly well for high level characters because their powerful magic items and prowess in combat is of less value. They cannot fight at every skirmish, lead every unit or fight every enemy incursion. This allows you to restore the initiative to the enemy, even if that enemy is compromised of simple soldiers who would be annihilated in a face-to-face battle with one of the heroes.