I agree with your last point, it would be nice to get some official reasoning why the war broke out (which may or may not be the same as the real reason). However, don't you think that it is bit hasty to conclude based on one DoW which seemed to be irrational to you, that diplomacy is entirely broken?
By the way IMO you just captured one of the eastern polish provinces (Vilnius?) which might be amongst the Russian victory conditions. Also, would not you be afraid of a "friend" who keeps conquering its neighbours?
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