Grief.

[Graft]

I was thinking of a system like this;

A steals from B, stealing is an act of war so B can either declare war to A (& rape their sh*t) or report the crime & A gets a penalty served by a mod.
A wants a piece of B's land/item(s), so A declares war. B can accept & go to war or deny and give up the disputed land/item(s).

Declarations of war;
Stealing
PvP
Formal declaration on the forums

Maybe divide the map into 2 parts, 1 part with strict no griefing, stealing & pvp rules and the other moderated griefing, stealing & pvp. /boring

Griefing sounds bad, it is. I'd like to think of griefing as the destruction of a person's creations with no gain. War however has a gain, but destruction during war should be realistic. So no floating blocks and random destruction. When you want to defeat a nation, tear down a house/wall/keep and leave some good lookin ruins!