Alternate origin for Eusdria
by Marc Saindon"The history of Eusdria is very similar to that of its neighboring kingdom, Robrenn (see this column in DRAGON issue #177). The Eusdrians are descended from northern barbarians dislodged by Huleís armies. They relocated in this southern region, hoping one day to reconquer the fatherland. That day never came. Eusdrians came to love this new land anyway and eventually established a medieval society." Dragon Magazine 178, p.44
Let's simplify things. Thyatis colonized this section of the Savage Coast with similar Germanic people from Hattias. Part of the deal was that they would be allowed to fortify again and reclaim their ancestral roles as warriors. A large contingent of Vyalia Elves also tagged along, providing Eusdria with a lasting elven influence.
Because of their common Hattian heritage, Eusdrians and Heldannic Knights tend to share similar viewpoints, although Eusdrian nobles might be concerned with increased influence of the Heldannics over their own local authority, so it's not all "Super Friends" here.
The relative peace with Robrenn, despite occasional clashes, is mainly to keep Robrenn as a buffer zone against the Humanoids further up north.
(Map from: https://mystara.thorfmaps.com/tag/eusdria/)