Alliance War is introduced in 8.5, centred around the 3main cities, it is a MCC1 pvp event. Clans which are either Royalist or Republican first join their respective secret societies, and with that reap some of the benefits, as well as a few downsides for doing so. Neutral clans, aswell as clanless people can join the mercenary faction, and they will be randomly distributed during alliance war if they want to participate.
Joining a secret society/ mercenarys(you have to participate in Alliance war to get the fee waived, and attempted prison the joaqin) is required if you don't want to be extorted when doing the Byron Clock Tower raid, as people who are in a secret society/ mercenarys who have participated in alliance war get a 1 ele jewel fee waived which you have to pay EVERY entry, thats a whopping 10mill per try, aka just the minimum to get lionelle would be a whopping 80mill, for each person, and that is not counting key crafting costs, and CS items used in clock tower.
also being in a secret society will allow you to participate in the updated LTDA war wat the usual faction fight time, with improved buffs for the winners. A downside for being in a secret society, is that you will be autored to people in the opposing secret society on certain maps(will confirm which maps soon), even outside of CW.
People who particiate in Alliance War will earn points for certain actions, and these points can be spent on some pretty neat things, most noteable in sGE is Golden Rosario, aka rosario with 1dr base.
Credits to : http://ge.iahgames.com/news/newsArticle.aspx?id=1677, and **** of sGE