Military: War
The tension between the three great cities of Requiem finally broke when Cognance and Penance united against Sigil, claiming that the Church of Triumvirism had been unfairly favoring them for decades. Whether or not this was true, the hard reality was that even combined, the two cities were not militarily strong enough to take on the powerhouse that Sigil's army had become. What's more, the alliance between Cognance and Penance was tenuous at best and quickly deteriorated when they did not achieve immediate victory over their common enemy.
What followed was nearly two decades of entrenched warfare with no side truly able to overtake the others. Eventually, the fighting died down as each sides enthusiasm for bloodshed was diminished. A true peace was never brokered however, and each side remained some level of hostile to the others until the Church pulled a move that managed to piss off each of the three cities just enough for them to overcome their differences...