The trade routes becoming so profitable had the noticeable side-effect of leading to great wealth to those tariffs taken in the bordering nations that stood between Byshur and Ghervenia: Annastal, Syrizan, and Claustrum. The feudal society in Claustrum did quite well, the king established several noble households which helped control the burgeoning population. Syrizan was a small kingdom who’s population had a fled a cruel leader in establishing Ghervenia. The original king had long since died, but the kingdom grew more unstable with the success of Ghervenian affairs. Within a decade or so, the kingdom had collapsed into a chaos, but quickly recovered into a republic under the leadership of a charismatic man by the name of Farr Ustan. The republic was able to quickly line its coffers by establishing tariffs, which gave it the legitimacy it needed to control the country. That being said, there were many in the country loath to become to attached to the new leadership having just overcome the monarchy. The important thing to understand here is that Syrizan was unstable, and not yet fully unified. Their ties were weak with the surrounding countries because their own country was so new. You can read more about Syrizan and its history here. Annastal was a farming country without much coherent government. It was well known for its large number of Gnome-Halfling villages, which supported a large export of foods, marking it as the breadbasket of Byshur.
The precursors to the Claustrum War were largely due to the king of Claustrum, Genn Opine Korchev the Third, and his unfortunate rise to power. His father, Genn Opine Korchev the Second, was killed by a Dwarven assassin when Genn III was only Thirteen years of age.