Involving players in a war is a pretty sure way to ensure a high combat session or three. The problem is dealing with effect of a few players in a very large battlefield. This can be done, but limiting players to attempting "key objectives" is a good way to control the play. This approach is certainly helped by a scenario that puts the player characters under the eye of a senior NPC. Outlined below is an example of players involvement in a short war. In some parts the players will be caught up in events; in others they have options not to participate. The events assume both player participation and success right through and is offered as a possible model that can obviously be modified and improvised indefinitely. Note that in fact much the player involvement was from their own ideas. I would always present the situation and see what they do with it before offering "missions" in guise of a commander. While the events will unroll whatever the player's action really, it should help their enjoyment to give tons of feedback on the effect of their actions (good and bad) so they feel in the centre of activities.
1- First action: assumes the players are mounted. As they approach a road in a border area, they will notice the odd refugee and ragged army groups on foot moving away. A grim-looking senior army officer or noble is driving two carts loaded with incendiaries (oil, naphtha, resin-soaked straw) against the flow. Questioning anyone will tell of a massive invasion that has pushed in the border garrisons. The cart commander will request the party assistance, since they are mounted, in destroying a bridge over a nearby major river. The idea here is to give the party exposure to a key NPC. The bridge is easily set ready to burn, but the commander will delay the firing till last minute, allowing as many as possible of the fleeing garrison troops across first. The border itself is a mountain ridge and in one to two hours, companies of the invading army will be seen in the distance on tops of the foothills. A largish company of defenders, well armed and moving in good order is sighted but suddenly a company of the cavalry from the enemy vanguard appears. A race to the bridge ensures with the commander uneasily preparing to fire it. When only hundred metres away, it is clear the race will be lost and the defenders turn at bay to face the cavalry. Alone, they are outnumbered and lost, but they look capable ... ? If the party goes to assist, they may well swing the balance but the commander will definitely fire the bridge rather than risk it being taken should the fight go badly ... After 4 rounds of combat, the enemy will suddenly be aware of the risk of the bridge and try to disengage so as to rush it instead.
2- Under siege: The border defences have fallen back on a powerful fortress, built in the rough terrain of a mountain range (other side of the valley), protecting the road to the capital. The enemy army cannot forage through here so must neutralise the fortress to protect the supply line. The commander of the fortress however has discerned that the enemy has split with a force going down valley and the long way round to take the capital by surprise or at least prevent relief of the fortress. He decides to take a large force of mountain- hardy locals through less-known routes to harry and hopefully stop this thrust but this will leave the fortress very lightly manned. Players will not be locals so will remain in the fortress. The man they helped at the bridge now commands the fortress and will request them so they can help him with any ideas to make it seem the fortress has more men than it really has. The enemy army has been delayed crossing the river but all too soon they arrive. The fortress' outer wall has no moat but is too high for scaling ladders or grapples. The gate is both powerful and a cunningly made death-trap. It opens into a gated courtyard that would quickly be a killing ground if the main gate is forced.
Early stage ideas: Players discover enemy magicians using some levitation or flying power in an attempt to fix ropes on the wall. A magically- or psionically- powerful party might like to battle enemy sorcerers from spying, attempting to kill the commander etc. Enemy might attempt parley with some bribe the GM knows might tempt the party.
Serious stuff: With no easy way in, the enemy gets constructing. Siege towers go up which are well protected with fire-proofing to the front (ie water-soaked wool, constantly dowsed). There are not enough forces inside to properly defend the wall from these so this is serious. After anxiously watching a few days, the commander decides a night-sally to fire them from behind is needed as they near completion. A very powerful party might attack several, otherwise they will be one of several parties sent out at midnight to attack. If one of several, they will have to address coordination of the attacks.
3- Relief: The old commander's gamble pays off and he successfully grinds the flanking force to a halt and by message has warned the prince of it. He now hurries back, while the prince sends most of his cavalry to engage this much- delayed enemy and is able to gather a large relieving infantry force to aid the fortress. The fortress gains hope from sudden movement in the enemy camps as a defensive line is marshalled at right angle to the fortress to meet the threat. Battle is joined but the fortress takes no part to begin with to avoid risk of losing the gate. A sally force is prepared though and party is expected to be in it. The arrival of the old commander with the remnants of his force at midday forces the enemy flank so their line is slowly turned with its back to the fortress. The enemy standard is right is front of the gate when the Prince launches a furious attack on the centre. It is time for the sally. The players are detailed to bring down the standard, others will chase the enemy general. For a powerful party, the standard will be defended by enemies champions. The standard will also be protected by anti-magic spells and possibly a duty sorcerer. If the players succeed, then the enemy army will collapse into a rout, though the Prince doesn't have cavalry to exploit this much. If they fail, the enemy will withdraw in good order though this probably wont interest the player characters much. :-) This scenario throws the players into a full-scale battle with a specific goal and few gaming systems have rules for this so here are my ideas. The trick for the GM is to create the battle about the players in an interesting way without setting up a wargame table. I think the "fog of war" makes this possible - the GM only has to describe the action in the immediate area about the players. You can use the following table of results for a 6 sided dice to help lubricate the imagination, thrown every few combat rounds.
Back to plots ...
4- Impasse: The enemy has recrossed the river further down and linked with remnants of the abortive flanking attack. They are growing in strength as reinforcements arrive and rafts are constructed. Neither side can easily attack the other across the river. The enemy has set up in a patch of high ground on a river bend and the ground both up and down river is mostly swamps and marshes, providing secure flanks. However, the swamps also preclude any foraging so the enemy is dependent on the supply lines through the mountain border. The Prince needs to dislodge the enemy from this ground though he suspects the swamps will bring disease into the enemy camp before long but he faces the same risk. It is decided to send spare strength across the river in small units to attack the supply line and reinforcements. The party is asked to be one such group and attack the supply line for as long as they can do reasonable damage safely. The first part of the trip once over the river higher up is to avoid enemy screening cavalry though these will be thinly spread. Increase the chance of a siting by day compared to night. The rough hill country leading back to the border will provide many suitable bases in form of caves (which may house the odd monster) or secluded bushy glens. (Describe the country to the party and then dice for finding a suitable occurrence every watch). A good map of rough hill country will help enormously if you prefer to play this more detail (better still, use a real map of an area you know well as this makes the description much more vivid and helpful)
The supply line will at first be very lightly guarded. The party might encounter in a day: say on a D6 (adjust for strength of party).
After a week of raiding, the supply line will get better guarded. In second week, the reinforcements will travel with the wains, so matrix could be:
On return, they should find the enemy has succumbed to poor food and disease and has pulled back with the Prince preparing to pursue. Another battle could be fought in the hills, weighed heavily in the Prince's favour, if the party hasn't done too well.
5 - Victory: This scenario is for a swashbuckling style with fast combat and more concern for fun than realism at its deadliest. The enemy invasion is broken and has fallen back inside its own border but the Prince has decided to press the attack to annihilate the threat once and for all. The remaining enemy army is now besieged in a fortress town, just inside the border while the Prince demands handing over of the leaders and laying down of all arms. He judges he probably has enough strength to carry the walls by assault though the cost will be high. They learn (a prisoner, traitor, magical?) however of a drain leading from inside the fortress into a moat that protects part of the wall. It is large enough for a person to crawl through but involves swimming underwater to its entrance and making the first part of the crawl underwater. (This is possible on one breath but should require a difficult skill throw to avoid panic). An initial scouting will reveal that the other end is blocked by an iron grille and patrols move past the entrance very regularly. However, the Prince is planning a pre-dawn assault on the walls anyway, and it seems that a party could slip out of the drain unnoticed in the confusion of the attack and hopefully open the gate. A certain element of trust is probably going to be necessary here for the player to take this on :-) - perhaps they would prefer siege ladders and burning oil? Best of all is let coax the players into thinking up the scheme themselves. (ie. they can be the bearers of the information about the drain to the prince, discuss it with "him," etc). Assuming they take it on, they will need a means of opening the grille which should be provided by the Prince if the party has not the means. A means of breaking iron will come handy later too. The wall has a structure of buildings on the inner side providing rooms for archers to use arrow slits, stores of defensive equipment, stairs, access passages and barracks. A good map of these (making up three levels, mostly one room wide, two at the base) is needed. The drain grille will open into a 'room', three sided and open to the inner court where the sewer ditch comes in. No access to any other rooms in the level. It will be a reasonable distance from the gate. The gatehouse itself will be on the middle level and accessible only from an internal passage past barracks on this level. So how do the players find fun instead of sudden death for their characters? My approach was to play this as a series of running fights, with the players thinking up every means of deception they could and thoroughly inventive spell use. It is dark and confusion reigns with people running everywhere. The players will encounter various groups soldiers, newly waken, rushing to man the wall on the most part, parties carrying supplies of torches to help light the wall, slaves carrying barrels of oil for throwing on attackers, messengers, comrades assisting wounded off the wall etc. On encountering enemy, they will automatically assume that part of the wall has been taken. They probably will fight but only briefly if the party is getting upper hand, whereupon they will turn and flee, calling for reinforcements. The party should be forced into every trick in the book to delay or ward off pursuers - give the party plenty of feedback that these are working. If they adopt disguise, then they should encounter a captain who tells them to follow him - away from the gatehouse :-). Of course, unless you have decided the enemy in non human, then they probably will be mistaken for friends anyway unless they announce themselves as enemy.
The gatehouse will only have at most two occupants - they weren't anticipating needing the machinery at the moment! The gate itself is a counterweighted drawbridge, operated by a chain windlass. It will take some time (say six rounds) to lower the gate by windlass and it is not much use till it is completely down. The defenders will notice the moment it begins to lower and the party will find things very hot at the gatehouse door very quickly. Of course, if the chain holding the counterweights is broken, the drawbridge will open very suddenly. A picture of apparatus might help your players. There is but one entrance to the gatehouse which probably will be crowded by enemy with the gate down, but players could squeeze through the gatekeeper's watch window - a 20' jump into the moat. If any of your players fancies a glorious character death then now is probably a great moment.