I, uh, don't think that's true. Everyone's been talking about how a land invasion of Marienburg is a huge pain and that we don't actually know its treaty status with Ulthuan. The wiki says the elves were only instrumental in fighting off the last expedition sent against Marienburg, and they handled the first two by themselves.
Marienberg is, in part, an elven city ruled by the Phoenix King.
The way I see it is that from the elven perspective Marienberg is a client state of Ulthuan, the local tributary around their city. That's not particularly accurate, but would probably look like it is from the High Elf perspective.
In this way of looking at the world, this is an attempt to remove Marienberg from Ulthuan's sphere of influence and place it in the Empire, or worse the Karaz Ankor's.
This has echoes of the Great Game. It will look a lot like the first step in a dwarven act of aggression to expel their remaining influence from the Old World. They're likely to be required by internal politics to support what they believe is their client.
On the agreements, the Empire signed and Marienberg has also agreed to the Treaty of Amity and Commerce in canon. This includes such wonderful provisions as an agreement of extraterritoriality for all crimes involving elves (including non-high elves), removing them from the jurisdiction of the Empire or Marienberg's courts and instead making Ulthuan responsible for trying them if they commit crimes on the continent. As a side note, by delivering the dark elf to the embassy, we were actually just complying with the Empire l's treaty obligations. If a human or dwarf kills or harms an elf in the Empire that also falls under Ulthuan's jurisdiction, and they are obliged to apprehend the suspects and hand them over to the high elves.
That may have interesting consequences down the line, as it means that if Ulthuan, for example, sends elven agents to murder dwarven caravans inside the Empire, the Empire has no authority to do anything to them, they're obliged to hand them back over to Ulthuan for them to impose justice.
The relationship between the Empire (and even more, Marienberg) and Ulthuan isn't one of peers. It's very directly and deliberately inspired and modelled on the one between the British Empire and late nineteenth century China. The correct answer here may well be to petition the high elven Empire to keep their other client in line.
