Slaughter of Harrenhal

Known as the Slaughter of- or Slaughter at Harrenhal, this exceptionally bloody conflict occured not during but after the First Clash. It was a war between the Trident Union and the Crusader Crownlands, sanctioned by King Jon Arryn II who was both hated and admired for this reckless move as it dealt with the commonfolk kingdom of the Trident Union at the expense of putting thousands of lives on a threadmill of carnage on both sides of the conflict. The dispute was initially because of the territorial dispute over the fortress Harrenhal, which had set off the fighting of the First Clash of Kings but after the ascension of King Jon II quickly dissolved into a war of ideology. This meant that both sides were incredibly eager for a fight and were fighting not for conquering but for absolute annihilation of the other. The deathtoll among both the armies and leadership caused not only the Trident Union but also the Crusader Crownlands to collapse shortly after the battle had ended. This was because not only Joseph Dondarrion and the Crusader Queen found their end in this battle, but also King Jon's own son who died brutally at the end of a commonfolk pitchfork. This instantly destabilized both regions involved and the rule of the new king who now had no direct heir. Some could say that this battle was the catalyst of the events that soon led up to the Second Clash.
Who knew that putting two extremely ideological states at eachothers throats in a legally approved war could result in such carnage?