The war was a background item up until that point. It was a source of tension, sort of, and it gave the setting a bit of depth. But when the boy Sarah had begun to get along well with at the Technologists' School declared to me that he needed to go off to war, I was taken aback.
"What do you mean, you're going to war? Sarah needs you! She's the protagonist and she'll need your help in the coming pages."
"I'm sorry ... I don't mean to inconvenience you, but you see, it's for my father. He would want me to continue in the family tradition, and he's not really all that happy I'm here at Technologists' school instead of training to be a soldier. Now that the war's almost on our doorstep--"
"But you love technology. You're good at it! And ... I hadn't exactly intended to write the war in this story."
"I don't mean to tell you your job, but perhaps then you oughtn't to have written a war into the background in the first place."
"Augh. Alright, point. But--"
"Sorry, I've got to get my things and get on the train. Hope things work out."
And so he went off to war, leaving me with another plot thread to write, which is a good thing in NaNoWriMo, as it adds words, but was completely unexpected.
Can you force your characters to go where you want them to? Maybe. Personally I've had little success doing so. Characters shoved into plots they're not interested in fall flat and grow dull. It's as if their life force, so to speak, drains away. I'm reminded of the game Pikmin, where the little Pikmin creatures' colours fade to almost nothing when they're not being controlled by the player. They droop and stand around idly, doing nothing, when they could be putting effort toward a worthwhile task. That's how my characters feel when I try to manhandle them. So I don't. I let them guide me. And sometimes I get some very interesting things out of the process.