They made a claim, not a declaration of war.
We did not counter their claim with a declaration of war either.
So yes, a sneak attack. We were until their attack, at peace.
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Just to point out, we made a claim in your territory... and then, well, nothing.
So assumed we'd not be at peace, since we claimed something there, in direct violation of your claim.
It is debatable (to a point of course) whether it was our responsibility to contact the 22nd and clear up the issue or the responsibility of the 22nd to contact us, since you actually were aware of it. Something else we weren't to sure about, mainly due to the lack of responds here we´d figure you´d make a plan and then remove us or such.
Myself, I'm pretty new to strategus and so far, well, all we got were random sneak attacks so I figure that's the rule of the game, rather than the exception, but diplomacy can be a fun game too.