but there is buildup, it's not out of the blue at all
the reasonings are realistic AND petty "you've killed our king" and "you've condemned my entire race to death", are two pretty good reasons for war, there's also the good old, "we have no more reason to play nice since the legion is gone, and we've hated each other's guts for over 30 years"
if anything we temporarily stopped having war because of the legion, not the other way around
having to hold hands for the current *insert big bad villain name here* every expansion gets much MUCH more boring, than going into open war once or twice.