The Lost Civil War is a conflict between two rival factions of The Lost Brotherhood Alderney Chapter; one led by Club Secretary Brian Jeremy which opted to remain loyal to Chapter President Billy Grey while he was in prison and another one led by Chapter Vice President Johnny Klebitz, who believed (quite correctly) that Billy's recent leadership had only damaged the Club's interests. The war was fought in 2008 during the events of The Lost and Damned.
The war began almost immediately after Lost Brotherhood Alderney Chapter President Billy Grey got arrested in Chinatown by the LCPD. Club Secretay Brian Jeremy believed that the club Vice President Johnny Klebitz had betrayed Billy to the police in order to become Chapter President, so he gathered his own faction loyal to Billy and started the war against Johnny's much smaller faction.
The faction led by Brian split from Johnny's and instead of hanging out at the official Lost MC Clubhouse, they opted for going to Brian Jeremy's Safehouse. The war remained cold until Brian decided to eliminate Johnny and take over the Chapter until Billy got released. In order to kill Johnny, Brian lured him and his three lieutenants Jim Fitzgerald, Terry Thorpe and Clay Simons to a meeting that was suposed to be a peace negotiation, but when Johnny and his friends arrived there Brian and his men attempted to kill them. This backfired on Brian since Johnny and his crew killed his goons and he was forced to go into hiding.
The two factions of The Lost kept having small armed conflicts for a while until Brian approached Ray Boccino of the Pegorino Family, to ask for support against Johnny in the war. Boccino, who saw Johnny as a better business man than fanatic Brian, betrayed Brian by telling Johnny where he was hiding. Johnny proceeded to got to Brian's safehouse and stormed the place, killing Brian's guards and eventually cornering him in his room. At this point Johnny either kills Brian or lets him escape.
If Johnny didn't kill Brian at his safehouse, he eventually encounters him on the street and Brian leads him to an Angels of Death ambush, but Johnny ultimately finishes off the AoD and Brian as well; so Brian ends up dead even if the player initially spares him.
The war ends at this point due to the destruction of Billy's support and Brian's death.
The casualties of the war leave most of The Lost Brotherhood's Alderney chapter dead and several of their business interests ruined. Subsequently, the deaths of both Jim Fitzgerald and Billy Grey along with the destruction of the Lost MC Clubhouse by Ray Boccino's goons leads to the dissolution of the Lost Brotherhood Alderney Chapter.