Clan wars are always an interesting area of MMOs, sadly most games don't practise this sort of combat to the full extent that it could be intergrated.
I believe that the only game that allows for real "clan versus clan" combat is Eve Online, as someone previously stated. In most other games guilds are formed which mainly revolve around PvE combat, even if these guilds do attempt PvP combat they do not have the objects around them to make a real "war" feeling such as structures, land masses. Essentially sovereignty.
Being that Dawn of Fantasy is a MMORTS, I would take the risk of saying that clan versus clan combat will be realised to good extent because of the core components of strategy games.