Truth be told, I don't really fault the two kings for their wariness regarding Almus (at least not yet). For starters, in their world where it's literally "might makes right", Almus' kingdom is in a better position then theirs in terms of military, economical, and technology progress which is shown as they won two wars (repelled one invader while significantly destroying the other), and quelled a rebellion in less than a week's time, all the which caused no noticeable lasting damages to the kingdom's standing. So just like that, Almus' kingdom is already looking like it's either going to become the most powerful and/or prosperous kingdom in due time.
Then there's Almus himself, the newly established king who participated in each of these major events and a driving force to the Kingdom's prosperous success. For those who know Almus, he's considered a great guy, extremely trustworthy, as well as someone who'll help people in need and won't hurt others unless it's necessary (usually self-defense).
However for those who don't know him very well, or only know rumors regarding him, he probably seems like a very ambitious person. For starters, he's (as far as outsiders are aware of) apparently the son of the mythical gryphon, which already suggests that he's not a normal person. Then he got mixed up in the kingdom's conflicts and in one battle he personally slew the commander of the enemy forces. Afterwards, despite already being married, he challenged the fiancée of the Kingdom's princess and bested him in order to take her hand in marriage. Now whether it was for love, lust, or desire for power, I doubt many of them know, however what is known is that this opened Almus' path to the throne and allowed him the right to rule. Not long after that, he ended the rebellion, in which the enemy's commander (the princess's former fiancée) died horribly and mysteriously.
So yeah, from an outsider's perspective, you have a kingdom that was filled with internal strife; calm said strife, pick itself up, boost it's economical and military power immensely, all in less than two decades. And then the new king who while seemingly peaceful and kind is also willing to engage in conflict and war in order to obtain something if he desires it enough. As such, assuming that's the image they have of Almus, then it's understandable that they're wary of him as well as willing to bolster their military since they can't guarantee that Almus won't just invade them. It's only when they start instigating Almus and/or make preparations to invade Almus' kingdom that their actions become less understandable and more foolish, assuming that Almus hadn't made a move against their kingdom to even warrant such action.