The peace society

A strong disadvantage of long-lasting peace is that, should anything happen which makes this come to an end, barely anybody knows how to fight and drive back the forces who've broke it. Peace has become a divine-given daily self-evidence - and the loss of it marks a huge trauma.
Not just that humans have no mental resources to handle and process that - they'll also be run over by the first and even most ill-equipped gang that comes along the way.