Civilization V: The Celtic Chronicle, part 2 (1150 B.C. to 520 A.D.)
5 B.C.: Missionaries from Cardiff are sent North to convert York and London to PC Elitism.
I may still be bound to peace, but that doesn't mean I can't weaken their defenses. By establishing my religion in their remaining cities, I'll even further trivialize their capture when I renew the war.
25 A.D.: The French and the Swedes go to war. The Celts grow wary, realizing they are the only civilization that the Swedes have not yet declared war on.
55 A.D.: PC Elitism becomes the majority religion of York.
70 A.D.: With the faithful PC Elitists of York crying out to be liberated from the unbelieving English crown, the Celts mass across the border to take the city. After 180 years of uneasy peace, the second Celtic-English War begins.