Umm...you are rival Gods trying to take over a planet, where does diplomacy fit into that theme?
As a matter of RP, the rival pretenders are all going to hate each other and want nothing more than to see the others permanently eradicated, so I fail to see where this fits in (except in MP games where you want to gang up on someone).
On the topic of declaring war, I have had war declarations on the same turn as an enemy cast "seeking arrow" or some other distance attack spell, which makes sense. But the declarations that seem to come out of nowhere, for no apparent reason, I have yet to figure out. As Gandalf stated, I've had them declare war on me, don't even bump into his provinces until way later in the game, and even then the provinces are empty and unprotected. So it seems like he/she/it was just spouting nonsense.