Perhaps the moot realizes that this is the only chance they have right now to open up there western front.
Or they will never win a war against Aurvendil ever again, because by the time Falkirk falls, Fissoa would be wise to sue for peace with Aurvendil.
Well, the Moot has nothing to fear from Aurvandil really. Aurvandil has had ample opportunity to war with the Veinsormoot with the intent to see it destroyed and we've pointedly refused to do so, and refused to escalate hostilities when we thought peace was attainable. Before this entire war happened, Aurvandil was going to march up north and to help Terran and Asylon, but we couldn't do so whilst Barca was threatening future wars. It was never our intention to fight D'Hara or Terran, and it still isn't.
Falkirk won't fall, I don't know what convinced you they will. You can't harm any part of them that matters, which is Madina Gardens and Madina City (which is why your war policy so far has been a disastrous failure as it involves looting their rurals which had no peasants or production anyway) Aurvandil sells them all their food, and Falkirk has what... a 14kcs mobile army plus militia, plus Aurvandilan support which if we deployed all of our forces right now, is over 30kcs mobile. Which combined is far more than what D'Hara, Fissoa and Luria Nova can combine to send against us, plus Madina Gardens and Madina City have high level fortifications. In months of war Fissoa is further from beating Falkirk than they've ever been, a single Aurvandil raid of Fissoa was more decisive and damaging than any other event in the war.
And if Falkirk does fall? Well they will just rejoin Aurvandil, and I would love to see how powerful we'd become with 90 or so nobles. The best choice for a power balance is to keep Falkirk around, particularly as Falkirk and Aurvandil hold each other in relative contempt due to Falkirk being a Freestate, and Aurvandil being a Monarchy (which, Freestatism decries as tyranny) and as mutual adversaries when it comes to knighthood and Chivalry which will inevitable lead to war just to prove whose chivalry is superior.
