[Chazar]

I think a Dominion-style alliance system should be very simple and very basic. Like this:

Each player should have an simple on/off switch for each other nation.

Flipping the switch produces a normal message to that player on his next turn.

If the switch is on, then you will never fight against that nation, i.e. your own armies will immediately retreat if attacked by that player (as if routing). [i]Your own[i] armies will stay put instead of attacking a province owned by that nation, except if the procinve does not contain any non-hiding commander. In this way, you can make sure that you do not accidentally fight your ally, but can attack along a common frontier without too much coordination.

In addition, if [i]you[/]i are attacked by that nation, your PD will not fight, but is preserved for the new occupier. Unnrest caused by handing the province over to the new owner should be minimal. Castles are not handed over and treated as if under siege (storming is possible as usual - either for backstabbing or for handing over empty castles). Thus armies can move through each others territories without causing too much damage (i.e. loosing PD and causing lots of unrest.) Gems, Items and Gold can already be sent through the message system, and that is enough in my view. Resources and supplies cannot be moved inside your own territory, so you should not be able to sent it to your allies either.

Best of all: Instant backstabbing is possible, just flip your own switch! After all, the switch only affect the behaviour of your armies! Furthermore, you must trust your allies that they flip their own switch when they have promised to do so, so a full alliance means that both have set the switch to "ON", and reacting to backstabbing takes a turn!