Well technically it depends. The SOLIDER do not like the Purge. Tactically speaking it would be more advantageous to strike the enemy when he's busy destroying your vampire enemies. Especially when the vampires are preoccupied killing the Purge and not killing the SOLIDER as much. Hence it's a tricky bit. One could argue the SOLDIER would back off, but then again the Purge is also a threat.SOLDIER would have buttered up the Purge a bit before Magnus' crew attacked, plus, the Purge were fighting on two fronts and caught off guard. I guess it would depend on how we want to play it. Perhaps Magnus himself could turn the tide, if shikaru wanted.Sure, but even if the Purge lost half their numbers, one Crusader can take on ten Newbloods. Not counting the tanks. Furthermore, when the vampires attacked, the Purge ceased all hostilities towards SOLDIER (there would be no point). Presumably, SOLDIER would do the same. Once a greater threat appears, minor factions stop fighting. To the Purge, fighting Newbloods is extermination more than battle. They can kill newbloods with their mere hugging capabilities, and Newbloods, fighting with their bare hands or improvised weapons, are useless against them.