Setting up a false-flag attack

[Raz Dazzle]

I'm trying to come up with something for the game I'm running. Two powerful nations, Kingdom A and Kingdom B, are at war. B tried to conquer A, but now A is winning the war and advancing through B's territory. Kingdom A is power-hungry, though - it wants the neighboring Kingdom C to join in the war on Kingdom A's side, and maybe be annexed by Kingdom A. To do this, Kingdom A will launch some kind of false-flag attack on Kingdom C, and blame Kingdom B.

My problem is threefold:
1) what kind of attacks might Kingdom A launch?
2) how does Kingdom A fool Kingdom C into thinking Kingdom B is responsible?
3) how might I lead the party in finding out the real culprit?

EDIT: I should note that Kingdom A has a lot more magical resources than most nations - casters above level 9-10 are rare in this world but Kingdom A has several - and Kingdoms A and C are already friendly trade partners, so an attack on C by B could be justified as trying to stop trade to A.

[Amphetryon]

I'm trying to come up with something for the game I'm running. Two powerful nations, Kingdom A and Kingdom B, are at war. B tried to conquer A, but now A is winning the war and advancing through B's territory. Kingdom A is power-hungry, though - it wants the neighboring Kingdom C to join in the war on Kingdom A's side, and maybe be annexed by Kingdom A. To do this, Kingdom A will launch some kind of false-flag attack on Kingdom C, and blame Kingdom B.

My problem is threefold:
1) what kind of attacks might Kingdom A launch?
2) how does Kingdom A fool Kingdom C into thinking Kingdom B is responsible?
3) how might I lead the party in finding out the real culprit?

1)The targets most likely to get Kingdom C involved will be supply-oriented, or symbolic. If Kingdom C gets a lot of their grain from trade, for example, then an attack (appearing to come from B) on the transport of that grain could motivate C to join the war on A's side.

A symbolic target could be anything that the general population of C holds dear. Assassinating a minor beloved noble could do it. If C is a theocracy, assassinating one of the high-ranking members of their clergy could be sufficient. If there is a regular festival that C has every year, an attack on the festival could serve the same function.

2)The easiest way to fool C is for A to pay members of B to carry out the attack, preferably waving their flag, wearing uniforms, or otherwise clearly identifying themselves as attacking on behalf of B. If actual members of B aren't an option, then attackers who loudly claim to be either from B, or acting on B's behalf are still a solid method.

3) Zone of Truth. Ahem. Assuming the party doesn't have the means to determine the truth of the false-flag attack magically, the traditional (some might say "cliched") methods of finding the real culprit typically involve following the money, or finding information in the possession of the attackers. Following the money means discovering the person who paid the attackers, then who paid that person, and so on, until they discover the truth. Information in the possession of the attackers could be coins from A, or letters from A giving cryptic instructions on how to get paid/carry out the plan.