As ever in war and deception there are many possible people who may have wanted war between Osoerde and Mhoried, a few suggestions of people who will be considered the 'obvious' guilty parties and their 'motive'.
* Osoerde. Punishes Mhoried for supporting Elenie in the war of the brothers Dauota, Gets a chance to claim a few provinces while other locals are distracted by the Grey Duke, serves his master the Grey Duke by stopping Mhoried from aiding Elenie.
* Mhoried. Draws Osoerde into a war away from Osoerde’s base to punish it for interference in the Wyld war, gets a chance to claim Morcosoer.
* The new 'king' of Mhoried. Gets to slaughter much of the internal opposition, consolidate his position at the outset by showing a clear external threat, maybe even gets to expand his new realm.
* Elenie. Forces Mhoried into a full confrontation with Osoerde, Gets a chance to ally with Osoerde against Mhoried/vice versa, gets a chance to weaken an aggressive Osoerde/Mhoried and snip off a province or two, aids his vampire master by distracting two potential foes.
* Tornilen/Ghieste/Coeranys/Roseone. Weakens an aggressive Osoerde, gets a chance to snip off a province or two, get out of an onerous agreement, or win an agreement on favourable terms.
* Alamie/Cariele. Weakens an aggressive Mhoried, gets a chance to snip off a province or two, get out of an onerous agreement, or win a favourable one. Serves their vampire master.
* Grey Duke, distracts two foes, gets a chance to turn wrathful foes to Shadow.
* LPA. Gets to humble an over-proud patriarch or replace them with a more subservient pawn, Gets a chance to expand into Osoerde should Elenie do well.
* HA. Punishes an ungrateful Marquis, hostile ArchDuke or weakens an over-strong Mhor, possible gets to make Mhoried a holy land where church and land are one.
* MOC. War, sublime war! Gets a chance to claim Bhalaene as a holy land.
* HoA. Forments discord between Mhoried and Osoerde making it easier to claim Morcosoer at a later date
etc, etc.
All of course have excuses of varying degree, but someone is bound to consider each of them as guilty even if they don't dare say it.