From the poll, it looks like the Risk style system will be implemented for the first civil war. I wonder if the following rules would do for what we want. They are a little long, but we can regard them as separate from the core rules - sort of like the tournament rules or rules for an event.

Rules for Risk style PvP campaign movement

A. Land movement:
1. PvP land movement is by province. Any turn players who are in a state of war with another player (combatants), they may use PvP movement. Alternatively, they may move normally subject to the constraint that they do not enter an enemy province. Non-combatants (neutrals) move normally.
2. Combatants on opposing sides may not occupy the same province at the end of a turn unless one is under siege by the other. If units happen to find themselves in enemy provinces other than by PvP movement – e.g. at the start of the war – then the GM will “teleport” them to the nearest friendly province.
3. Each turn, a combatant may move each unit to an adjacent province. This is done by submitting written orders in secret to the GM. The GM will then simultaneously execute all orders.
4. On the campaign map, the GM will place units moving by PvP movement in or adjacent to the settlement of the province they enter or reside (it is possible to use PvP movement to stay within a province, but group up at or adjacent to the settlement). Combatants wishing their units to be placed at alternative locations in a province (e.g. to hold a bridge against the AI) should use normal movement.
5. Should orders result in enemy units occupying the same province, a PvP battle may be fought. The GM will inform all sides of the units occupying the province and then both sides should secretly give the GM their pre-battle option: (a) fight; (b) retreat; (c) retreat behind settlement walls (if the player controls the settlement)
6. All battles that take place in a given turn are resolved before any retreats are executed – even pre-battle ones. If there are multiple battles in a turn, they are resolved sequentially in a random sequence.
7. Retreats are executed by units retreating to the province they came from or, if they were stationary, to an adjacent province not occupied by an enemy unit (their choice). If there are no such provinces, the army cannot retreat.
8. Only 20 units can retreat behind settlement walls (whether damaged or not) – any excess are disbanded. Units can be merged prior to retreat. If there are any disagreements between allies on which units retreat, the owner of the settlement decides.

B. Implications of PvP movement for land battles
1. If units from one player enter a province occupied by units of his enemy, then the entrant is regarded as the attacker. In a PvP battle, the GM will give the defender some suitable advantage in terms of choice of terrain (e.g. first pick of map side in Multiplayer). After a battle, the attacker must retreat unless he defeats the defender (draws and standoffs are treated as victories for the defender).
2. If neither side began the turn with a unit in a province, then the battle is regarded as a meeting engagement. Neither side will have an advantage of terrain
3. Bridge battles: if on the strategic map, all attacking units side could only have entered a province via a bridge, the battle will be modelled as a bridge battle.
4. The losing side of a battle can retreat any surviving units according to A7 (but the GM implements this only after all battles are resolved)

C. Implications for siege battles
1. If one side has retreated behind settlement walls, then the attacker may assault the settlement that turn or besiege the settlement.
2. Besieged armies can only exit the settlement if all enemy units leave the province, if they win a sally battle or if a relieving army attacks the province and wins. The GM will specify how besieged armies can hold out before automatically surrendering and may model attrition, guided by how the game engine models sieges involving the player vs the AI.
3. If a relieving army enters a province, there is a field battle with the besieger’s positioned between the relief army and the besieged garrison. For simplicity, the settlement (and the garrisons need to exit through the settlement gates) need not be represented in the battle.
4. If a garrison loses a sally or relief battle, survivors may retreat behind the settlement walls at the GMs discretion. They cannot retreat anywhere else. If garrison is in good order after the battle (it sallied only to impose some losses on the attacker), a retreat is appropriate. If it is utterly crushed, the GM may just declare the siege won.

D. Naval movement
1. While PvP land army movement is by province, PvP sea movement is by port. Unlike land movement, combatant ships may NOT move normally. All combatant owned ships must start each turn either (a) in a friendly port; (b) blockading an enemy port; or (c) a transit area (represented on the campaign map by being placed somewhere in the relevant zone at sea).
2. Each turn, players can give the GM secret orders to move their ships to any other port in the same sea zone or to the transit area for an adjacent sea zone. There three sea zones: the Black Sea; the Mediterranean; and the Atlantic – Gibraltar and Constantinople demarcate those zones.
3. Each sea zone has an associated transit area. No sea battles between combatants can take place in transit areas. Enemy ships may share the same transit area. Ships may stay indefinitely in a transit area. (These are all abstractions made for simplicity).
4. If orders lead to enemy fleets being at the same port, then a naval battle may result. The parties communicate their orders: (a) fight; (b) retreat; (c) retreat inside port – if the port is friendly.
5. If opposing combatants both fight, then the GM will resolve the sea battle. All sea battles are resolved before retreat options are executed.
6. Retreat orders are executed by returning ships to the port they originated to, provided it remains friendly and not subject to blockade. If the ships did not move, or their port of origin is no longer eligible to receive them (e.g. it is captured or blockaded), they may move to any non-blockaded friendly or neutral port (note: in the latter case, they would become owned by the neutral!). Ships may not retreat to transit areas. If no ports are eligible to receive a ship, that ship may not retreat before battle and is disbanded after battle if required to retreat.
7. If one side retreats inside the port, it is blockaded. Blockaded ships cannot leave a port except to fight a sea battle against the ships blockading them. Such a battle expends their movement for that turn.

E. Naval movement of land units
1. Land units that begin a turn in province that has a unblockaded port with friendly ships may be transported by those ships (2 units per ship). The GM will disband land units travelling with ships if losses from sea battles mean there are no longer enough ships to carry them (which units are lost is randomised).
2. Land units transported by ship end the turn in the destination province provided only if any of the following conditions apply: (a) it has a friendly or neutral port that is not blockaded by enemy ships; (b) it has an enemy port unoccupied by enemy ships; (c) it has an enemy port under blockade by friendly ships. (This implies, as in Shogun, that an enemy province can only be subject to naval landings if it has port infrastructure – this is rationalised by supply issues. It also implies that the blockading side effectively controls the surrounding waters, facilitating or obstructing landings.)
3. Land units transported by sea to a province held by enemy land units are treated just as if they entered by land. They may retreat before battle (effectively a feint or threatened landing), but not after battle (think Bay of Pigs).