[quote=@Mihndar]
<Snipped quote by Willy Vereb>

Are you going to have your forces end up skirmishing with the Tin, as suggested by Voltus, or will it be a more muted reaction?
[/quote]I doubt I'll be fighting Tin unless it provokes me.
Bombings happened on both sides, causing equal damage. It's obvious that whoever attacked it was enemies with both Yllendthyr and Tin.
After analyzis it'd be determined that military grade explosives were used and the kind which may not even be standard for either Tin or Yllendthyr.
So yeah, this whole plot is actually rather transparent.
Somebody either infiltrated Zanshir islands or supplying local rebels with weapons.
The question is who?
If the explosives are something Yllendthyr uses then it could be a rebel element in the local Drefgiad.
If the explosives are foreign then my first suspect is KalMea because I know they deal with advanced military grade explosives.
I could also suspect Avalia but they would only really attack Tin and they might be almost happy I have a presence in Zanshir.
And so on.

Generally it's just that a couple of bombs damaged buildings and killed people across the borders. It would certainly increase tension between the local forces but this alone won't be enough to cause war.
If Zanshir openly rises up against the two empires then it may even cause me to slightly cooperate with Tin for a while.
With hundreds of thousands of soldiers staging it's quite obvious that any kind of rebellion in the islands is bound to fail.
Long story short, you need more to actually cause a war.

I expected something more insidious which could've ended up with either side blaming the other. In addition I kinda overestimated the effects.
So yeah, Yllendthyr's response would be that another source of annoyance showed its head.
Whether these events later escalate into war or not depends on how all the other nations would react, IMO.