Three Kings' War

The Three Kings' War was a military conflict in the Kingdoms of Etrand and Froturn that resulted in a two-year regime-change in Etrand, and a permanent regime-change in Froturn. If we disregard the Etrandish intervention in the Froturnish Civil War of 809 AEKE, this was the last time in history that Froturnish and Etrandish soldiers fought against each other. This conflict was also the only time in history that Froturn succesfully invaded and occupied Etrand.

Background
From the Etrandish perspective, this war was a tragedy following another tragedy: in 102, Etrand's one and only knightly order, the Knights of the Golden Light, the were disbanded after being nearly completely eradicated during the infamous Knights' Massacre in 102. What followed was two years of fighting that finally led to the Expulsion of the Undead in 104. The Etrandish thought that they could finally breathe some fresh air, but no...

For various reasons, King Hereric I of Etrand had very low relations with Froturn. Hereric was enraged that Froturn decided not to aid Etrand during this time of troubles, while Froturn was getting uppity about Etrand acting "too independent", despite having never been formally a Froturnish vassal to begin with. The straw that broke the camel's back was rumours of Hereric's ambition to nationalize the church within Etrand, effectively causing a schism - "that's it, the Etrandish have to be taught a lesson" thought the Froturnish.

Little did both sides know, that Froturn didn't have much of a bonus of stability compared to Etrand either, Froturn itself having gradually and unnoticeably become a powder keg waiting to explode, with an economical crisis, a military that isn't as loyal as previously believed, and a stubborn ruling class that doesn't have what it takes to remedy the situation.

Aftermath
The rather devastating Three Kings War between Etrand and Froturn ironically ended up cementing the friendship between the two countries on the long run, as the two since have not gone to war against one another, with the exception of the Etrandish intervention during the Froturnish Civil War of 809.

In Froturn, the war resulted in a regime change leading to the purge of many officials, turning Froturn into a de-facto military dictatorship for more than a century to come. In Etrand, the war made it unambiguously clear that Etrand is a free, sovereign state, not a vassal of Froturn. While the rule of Hereric may not have been kind to the church, his successor respected the Yanus Protocol, leading to a complete normalization of relations between Etrand and Froturn, as well as the Church of Titanius.