 If you liked
Gungrave
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 ...then you might like
Ga-Rei: Zero
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Both series focus around a couple of people who have a strong friendship bond and would do anything to help each other until something or someone else interferes, with the resulting actions causing an immediate shift from best friends to enemies. Not to mention both series also start with the end and then show the events leading up to what happened at the start.
Originally written by Seasonreaper....
http://myanimelist.net/forum/?topicid=58150&pages=1&show=0#msg2469841
Both series start off in such a way that the viewer doesn't really have an idea of what exactly is going on; after which a flashback starts that lasts for a large part of the series and details the events leading up to the situation shown to the viewer in the first episode (first two episodes in Ga-Rei-Zero's case). Both series also happen to revolve around a deep friendship between two people that somehow took a turn for the worse, the how and why also being revealed over the course of the story.