Manual states: For the 'City Under Attack' subtype, the city requesting aid is being attacked by a rival army.
Use the 'City Attacks Rival' event when you want one of the player's allies or vassals to attack a rival.
The manual alludes to ally status being important but from experience, we know that 'city under attack' will fire regardless of ally status. The subtype 'city attacks rival' also seems to fire(when/if it does) regardless of ally status.
'City attacks rival' has a borderline situation where the event messages never fires, but during the trigger month the 'city summary page' and empire map will show the military request. I ran several tests setups(allied/not allied/military/no military) with feb+1 as the trigger and got consistent results exemplified by the save.
The best explanation for the disappearing request would be invasion called off due to discovery of a 'bad decision'(non-player feels its military is too weak). This hypothesis is supported by several variations of test where I extended the trigger month to July+1 and the request just doesn't materialize(not even for the trigger month). The tests I've conducted thus far are early within the missions(1-1.5 years) with all cities having equal military strength. I would gather that they would eventually attack city#0 (the requested troops go to the requesting city). but I have as yet not had a message to read.
Thinking that the 6 month leeway (troops take ~6 months to reach the water route requesting city) may be the problem I extended it to 12 months and tried another run at July+1. No joy, sort of confirming the 'invasion called off' theory.
[This message has been edited by shaun (edited 04-09-2004 @ 10:35 AM).]