Xander joined the Army amd was some higher up in a demon fighting section. He rescues Spike from some institution (the initiative?) and persuades him to join the fight. Xanders group feeds Spike their blood. There is a theory: a vampire feeds 3 times of you and you can be turned without draining you. Spike saves a couple of soldiers by giving them his blood.
Buffy and Willow are the evil ones in this fic. At least till it got out that Tara was taken prisoner by some evil men and repeatedly raped. Willow agreed to work for them if they let Tara go. She also takes Taras memorys of her capture into herself to help Tara.
Later on Spike and his childe are captured and tortured by Buffy and Riley.
Can you help?
PS: A while back I was looking for a fic in which Spike didn`t escape the Initiative. He became a Tracker with Riley as his Master. If you are interested, I found it again by accident at the Sandlot Fic.
It´s called "Seventeen, not so Hostile" by Lore.