You can look through the Official Lore Portal for various back stories and chronicles about the Yulai convention.
Basically, CONCORD, the “police” force, is actually more like The United Nations (UN), in charge of keeping the peace between the empires, and in charge of regulating “capsuleer” affairs. According to the lore, all the empires fund our training and our NPC station clones, in order to get the benefit of all the people in high-sec doing missions for them, or whatever.
As far as the war declaration, the enemy corp must pay a fee to declare war on your corp. Then that lasts a week, and they must pay a fee again to continue the war (it’s like a weekly subscription). They have the option of outright canceling the war, but if they don’t do that and just let the subscription go unpaid, CONCORD invalidates their war (cancels it).
That’s the message you got. The primary enemy cancelled the war, all the secondary enemies didn’t actually click the cancel button, they just didn’t pay the renewal fee.
By the way, if someone declares war on you, and you declare war back at them, then it’s a mutual thing and the war fees become “free.” Like a duel.