Well, speaking as the General in question at the time I don't recall the way forward with the issue of Mansbridge being made clear by anyone that had more substance than "you can trust MI" which had, over years of war, repeatedly been proven not to be true and in any case was being said by Norland's former allies who had recently betrayed them to side with MI against Norland on the flimsiest of pretexts (in a Norlander's eyes a royal loses his status when he lowers himself to murdering nobles in their sleep and should be treated like any other common criminal).
MI had promised to return Mansbridge to Norland once relations improved but were vague about the terms and refused to clarify them before the treaty was signed. Many in Norland wanted to refuse to sign the treaty until that was rectified because they fully expected MI to renage on this deal but the then King signed anyway. Norland repeatedly tried to raise relations, even offering to feed MI's starving regions but were met by silence. As MI started to garrison Mansbride and allow their allies' troops to pass through it (against the terms of the peace treaty which forbade any military forces in the region) it looked like they indeed had no intention of honouring the treaty. Given their history of double-dealing and political treachery there was no reason to trust them to.
MI had failed to honour the treaty so Norland took back what had been promised.
MI's new allies then obligingly wiped Norland off the map for them, incidentally gaining much gold and land in the process.
Whatever else was known, believed or talked about outside of Norland this was all that Norlanders saw.
Sure, we could have waited and unspecified amount of time to see if MI might do an about-face and honour their treaty or, when the threat of war in the south faded, use Mansbridge as a staging post for an attack on Norland but standing idle in the face of treachery and threat because it was safer to do so was not Norland's nature. When the question of taking Mansbridge by force was raised the reply came back from all levels of Norland and that reply was "fight!".
