Already we see now people are choosing sides and defending them zealously to the end. We as a whole will each have to choose our factions, but we all share the same forums, together. The implications could be enormous, especially if the inferior Dead Orbit scum keep think they are even remotely on the same level as New Monarchy.
[spoiler]I still don't know what faction I want join, lol[/spoiler]
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I think what would happen is when the game is out, people will start groups named after factions, as they join faction wars they'll set up a game where one bungie group named after a faction fights another with the same plot. Something as big as separating the whole community would be surprising.