I missed this tidbit from the forum discussions:

The demo capped off with a display of Elemental's equivalent of a nuke: triggering a spontaneous volcanic eruption under an enemy city that completely obliterated it from the map. That won't happen too often, Wardell assured me. "When you start doing this, everyone declares war on you, because you're using weapons of mass destruction."

I really really like that idea. Not only does it allow for the player to instigate a better challenge if he's walking all over a map, it also makes mop-up duty that much faster.

MMDs - Magic of Mass Destruction

+1 for your quote and the MOM forum avatar

I like in part, but dislike it being an "all or nothing" approach. Yeah, using WMD's is likely to piss off a lot of people, but not to the point of everyone else declaring war, no exceptions. The more evil races would likely be cool with it regardless, and others would see it as justified against those races, and regardless of moral leanings allies would be more understanding when dealing with a mutual foe.

If there other considerations, never mind,, but I've always hated it when games go that route, even mass murderers can have allies.

I think the fact that you can "at will" destroy any city on the map is enough of a reason for everyone to doubt your intentions.  At best you will hold the threat over everyone that you could do it, at worst you will proceed to slaughter all that will not submit to your whims.