Military action
Short war over magic resources.
The Rubefiàn emperor of the time was attempting to exploit a loophole in the Treaty of Masinià̃: less magical plants were equally less regulated. The islands in the Mau̯humel Archipelago all have useful plants that are low in magic content. However, Hache Island was inhabited, leading to an extended war.
The Guild put Rubefià under tariffs for ten years as punishment, as this type of incident is exactly what they were founded to stop.