I believe one of the goals in Strategus was to have people attacking on a regular basis. So to that end, I have a suggestion I think could very well solve the problem. Either a reward or punishment system for factions based on how often they attack another fief and how many fiefs they own.
Let's say a faction has ceil(25/numFiefs) days to attack or defend a fief (where numFiefs is the number of fiefs a faction owns) before some kind of punishment system kicks in. In this case, I was thinking something along the lines of a 0.5% troop decrease per hour for every member in the faction as less troops are needed in times of peace. Or maybe double upkeep as troops get pissed that they're away from family when there's not fighting. And it would just go back to normal next time the faction attacks a fief. For those factions that don't own any fiefs, 25/0 is infinity, so they don't have anything to worry about. However, I wouldn't leave it to a computer to translate it that way.
But since punishment systems usually result in anger more than anything else, how about a reward system instead? As long as a faction keeps up a rate of at least one attack every 25 days over number of fiefs owned, they get a bonus. Could be any number of things: increased recruitment rate, war time equipment crafting production bonus, less or no upkeep, etc.
What does everybody else think?