One turn I got three diplomatic messages from the only surviving faction I had contact with (Capitar?), in this order (ad libing the messages):

- Pay us <most of your treasury> gold to survive. (I declined)

- Would you like to make a trade agreement (I accepted and had to pay a small amount of gold to balance it in their eyes)

- Prepare to die! (Long war ensued)

It seems that if a faction is considering war it should not also be considering trade unless it completely breaks off all hostile intentions (at least for a few turns).  If this is correct functionality it is a sneaky way to fund their war effort.

Afterwards I had "Enemy in my Territory" of several of their caravans and got a message that one of mine was destroyed.