It seems clear that God reveals Himself as a God of Battles in much of the OT. So what of these NT teachings?
In this case, the NT references to peace are interpreted to be the antomym of 'war', when this is obviously not the case.
In Romans, Paul seems to be speaking of peace in a subjective, existential sense - a relationship with God brings a sense of peace.
In Corinithians, Paul is speaking about the activity of Church congregations - they should be orderly and peaceful, not full of confusion and contention.