Many many years ago I read an article about ‘Sideshows’ i.e. little engagements that would have an impact on the big picture. The example given was an ACW game of about company size somewhere not far from Appomatox Courthouse. If the Confederates win, then that is why Grant give Lee such generous terms, if they lose then that is why Lee surrenders. It give a reason for the two sides to fight.
For my own experiences on campaigns, and there have been many, I would recomend what I call the ‘Coat hanger’ method. e.g. Having aquired half a dozen dirt cheap out of date tourist maps of the island of Rhodes, we based a WW2 campaign on it. We listed everything we had (that was WW2:-20mm inf/vehicles,; 1/600 ships; 1/300 aircraft; 28mm skirmish figures etc) Each side (Allied & Axis) then wrote down where it was (e.g. 1 german destroer & 4 E-boats in Lindos harbour. 1 section of German infantry in the Acropalis of Lindos & so on) We did not fight and/or move everything (would have taken far too much effort) instead when we wanted a game we first decided what it should be (Air dogfight/ comando raid/ landing etc.) both sides knew what they had available but were unsure as to the enemy as they were allowed to exaggerate or conceal up to a pre aggreed level. This led to some very interesting games. The German Navy’s attempt to break out of Lindos was a complete disaster as they though the Royal Navy player was exagerating his strength to keep them bottled up (just like Graf Spee) but it turned out he was concealing a Cruiser ! In an other game the Allies planned a big landing on the Northwest coast, not far from the Airfield in an attempt to bypass stubborn resistance & capture the airfield. Their origional landings in the southwest hads been almost unopposed, (what can I say we didn’t have enough men/equipment to cover all of the possible landing sites) but stiffening German resistance had blocked their progress up the west of the island. The allied attackers were played by 6 different club members the defenders just 2. It looked like the defenses were strong (Trenches, pillboxes, MG positions & a dug in tank) but it was an exageration, in reality the trenches were empty, as were the pill boxes and the tank was a wooden dummy. Still the attackers were all very keen to not be the player to attack first. We had ammo & fuel restrictions (simply draw a card at the start of the game, as a result the attackers all thought the reason the Germans were not firing was to conserve ammo for short range, when some brave/stupid allied player rushed them across the barbed wire, which they were sure was mined. In the game one MG teamheld up over 120 Allied infantry, who had five 6pdr guns & 5 moves of naval fire support, untill two truck loads of Italian infantry could be rushed up from the Airfield. Much fun was had from this style of campaign, and it avoided many of the reasons previous campaigns had failed.