There are actually a few solutions, but nearly all of them will touch upon a subject of ownership. I'll try to list a few steps that can help you feel comfortable with their past.
Talk to your players.
Privately or publicly, whichever works, unless they are not sharing they backstories. Probe and ask until you have an idea about how much they care about their backstory, how deeply it affects them as players. You are saying that until now none of them provided you with such. That hints towards your players not caring deeply about this sort of things. Sure, they might now - and that you need to find out.
Even if just for yourself.
You are the owner of the world. If a backstory of one of the PCs describe a major event that nullifies the whole premise of war, would you go with it? Probably not.
Remember that you can amend backstories. Anything that would spoil your world in that 15 pages of bio? Talk to the player, explain what breaks your setting.
Do you consider backstory characters, like said teachers, families etc. NPCs or PCs henchmen? If latter, then you are effectively equipping them with a herd of supporters that the PC has mind control over. How would you establish a fratricidal feud against one of the PCs if said brother is controlled by your player?
Proceed with common sense and moderation
Your players are consenting adults, no pun intended. They are aware that during the war peaceful villages burn and it's not a GM's sadistic vendetta if it happens to their village.
Try to establish key characters, the ones that "drive" your PCs. They are the ones that you need to treat carefully. Ask your players a few generic questions about "What would your [significant character] do?". When handling those characters adhere to that, with exceptions (see next paragraph). The rest of the characters are going to have an impact on the PCs, but will most likely not break them.
Follow Situation-disturbance-resolution scheme
It's alright to drive PC's brother crazy and make him fratricidal, because he feels that PC abandoned his family (where in fact the PC was just late to the scene). Unless it's completely unreasonable, try to accomodate backstory NPCs into the story as major NPC (that you frequently act out) only if you change them in a way that represents the new status quo. That way you will be safe from acting them out wrong, but still be able to create a interesting situations
Stick and carrot
This gives you a great opportunity, especially if a player is attached to his backstory. My bet is on the PCs trying to "restore order" and "fix" the situation. This disturbance is actually much more compelling for the player that usual carrot. Ever tried to take away your archer's favourite bow? Ever threatened paladin with frown from his deity? Try, but don't go over. The idea is not that they have to lose everything. Threaten, make them uncertain. Make them consider their backstory going away without explicitly making it happen. News are the village has been pillaged. Beloved teacher once again drafted into the army and sent to the front. Brother sends a message that he deserted, now hiding. Make that something present, make the players consider that things at home are not going well, without actually executing their past. They don't care? You can play with those NPCs as much as you want. They do? Even better, take advantage.