jjsimp: Not to mention the attack through a friendly neighbor, that I just don't get. An ally should not let a wartime force through their country to attack an ally without turning against one of the two nations.
Why? It happened all the time in ancient / medieval diplomacy. They are your allies, but when they have an option:
a) be buttraped by your unified enemy
b) let them pass and buttrape you for exchange of non-aggression
I would choose B all the time.
It's actually pretty human behaviour. If i see a force gathering near my borders I simply can't defend against, I
always let them through, especially in games like EU3 or CK2.
Being your ally does not mean willing to die for you. Not in politics. By the way, it should be
you who declares war on your former ally because he betrayed you, not the other way around.