Disclaimer: If Naruto were mine, we'd see a lot more Yondaime and Sakumo flashbacks. Unfortunately, it is not, and we do not. Oh, woe.
What you don't know can kill you.
Kakashi's father had told him that once, when he was very young and rather foolish and more naïve than anyone had a right to be. It was bright and sunny outside, and the birds were singing in the cherry tree outside his window, and in the courtyard below he could hear the faint swish of steel through the air and the even fainter panting as his father practiced his katas. He didn't want to read about the signature Doton jutsu of the ninja of Iwagakure no Sato; he wanted even less to struggle through three (real) mission reports and write three essays pointing out where the ninja writing the reports had gone wrong. (His spelling was atrocious, and always would be. The Academy sensei had no time to teach spelling when theywere teaching survival and assassination instead).
After all, even a shinobi child needs an afternoon off.
But Hatake Sakumo catches him halfway down the stairs, arms folded, eyes narrowed, tanto dangling from his fingers. "You want to sneak off," he says, "you make less noise when you're there. I knew the instant you stopped kicking the table leg."
He pastes on that pleading look of wide-eyed innocence that only three months ago made his father grin and laugh and scoop him up for a piggy-back ride 'round the courtyard or a romp with the nin-dogs. But much has changed in those three months—too much. First the rainy day when Sandaime Hokage summoned all the ninja of Konoha to a meeting from which Sakumo returned tired-faced and tight-lipped, and then the weeks in which his father and his friends' parents and even the sensei at the Academy disappeared on long missions from which they came home limping and bloody and sometimes not at all. This is the first time Sakumo has been home for more than two days at a time, and Kakashi's old enough to know that the wide white bandages on his father's left bicep and right thigh are the only reasons the jounin isn't out again.
Sakumo's stern face softens a little at the pleading in his son's eyes, and he reaches out to tousle the unruly white hair. Then his hand slides down to his son's shoulder and he turns him around and gives him a gentle little push back up the stairs. "Get your homework done," he says. "And then maybe you can come out and practice your taijutsu with me."
"Can't we do it now?" Kakashi asks, a tiny edge of whine creeping into his voice. "It'll be dark by the time I'm done, and it's so boring. I wanna come out with you. Please, dad?"
His father hesitates, and for a split second Kakashi dares hope that the past three months and the missions and the bandages and the low voices that stop the minute he enters the room haven't changed the world that much. Then Sakumo's hand tightens on his shoulder, and the deep voice behind him says quietly, "What are you studying now?"
"Earth ninja," Kakashi says, lower lip pushing out in a pout he only dares because his father can't see his face. "They're stupid. Who wants to know about dumb ol' rocks?"
The hand on his shoulder clenches a little tighter and then falls away. "Come down into the kitchen with me," his father says. His voice is as soft as the silken sound of the tanto sliding home in the sheathe between his shoulder blades. Kakashi spins, glad of any reprieve from the dreaded homework, but Sakumo is already turning, heading down the stairs with only a little hitch to his step.
In the kitchen, he kneels at the table in secret glee while Sakumo makes tea and then kneels across from him. His face, bared in the privacy of their own home, is unusually serious. The little smile that always twitches at the corners of his mouth, even in these last three months, is gone now.
"Drink your tea," Sakumo says automatically, and Kakashi obeys while his father turns his own cup round and round in his strong blunt-fingered hands. The lines in his face are carved a little deeper than they've been at any time since his wife's funeral, which is one of Kakashi's earliest and most vivid memories. The kitchen suddenly seems a little less bright, the tea a little less warm. The birds are still singing outside, but there's a mocking note to the cheer of their song that Kakashi has never heard before.
"Sandaime-sama said," his father says, still watching the steam rising gently from his tea, "that we shouldn't tell our children. It may be over soon, and no need to worry; there've been scuffles like this for the past ten years, and no long-term harm's come of it. But…"
He turns the cup once last time and then sets it down on the table and laces his hands together. "But it's been three months now," he says. "And it's clear that there's no end in sight. And things are going to get far, far worse."
He catches his son's eye, and there's something in his quiet dark gaze that makes it impossible for Kakashi to look away. "When you started at the Academy, six months ago," he says, "you could barely even tell me what the word 'chakra' meant. Now you're studying Doton jutsu I'd never heard of until I was twice your age." He hesitates for a moment, then adds, "One of those Doton jutsu nearly killed my team four days ago."
Kakashi catches his breath in horror. His father's mouth twists in a new smile, a wry painful little expression that has nothing at all to do with mirth. "We're in a war now," Sakumo says, "and you children will be heading into it before you're old enough to wash the dishes without standing on a stool. That's why your sensei are pushing you harder and faster and giving you more work than I could have dealt with as a genin. They can't hold back. They don't dare hold back. Because the one piece of information they neglect to teach you is the one piece of information you're going to need, and if you don't have it, it's likely to be the fact that'll see you killed."
He reaches across the table again and tousles his son's hair once more. There's all the love in the world in that gesture, but in his eyes Kakashi sees not just love but hope and fear and pride and a terrible, terrible sorrow. "Your grandmother used to tell me that what I didn't know couldn't hurt me," he says. "But she was wrong. What you don't know can kill you. Will kill you, sooner or later.
"And so, Kakashi, all your teachers can give you is everything they know, and hope that it'll be enough."
"But you can't know everything," Kakashi protests, his hands clenching white-knuckled around his empty cup. "Even if I got as old as Hokage-sama there'll always be something I don't know."
Sakumo's smile is as slow and gentle as the hands with which he tucks Kakashi in at night, the same hands that have slid a tanto blazing with white chakra home into a hundred hearts. "That," he says, "is the most valuable knowledge of all."
Sakumo didn't know, when he made the choice to save his team and doom the mission, that the men he'd protected would turn on him with the mindless viciousness of rabid dogs. Obito didn't know, when he swore he'd save Rin and challenged Kakashi to do the same, that he was charging to his death. Kakashi's sensei didn't know, when he accepted the hat and the robes and the title that weighed him down more heavily than either, that in less than eight months he would die for his village.
What you don't know can kill you.
But Sakumo knew, when he thrust the tanto into his guts and prayed with his dying breath for Kakashi to forgive him, that he died to purge the shame of failure from his son's name and to give him a future clear of guilt. And Obito knew, when he knocked Kakashi out of the path of the plunging boulders, that he was sacrificing himself to save the friend he'd just barely won. And Yondaime knew, when he set his hands into the final seal and looked back one last time on the smoking ruins of his village, that he was dying for something more than a name, more than a building, more than himself.
Kakashi wonders, as he stands at the Heroes' Stone and gazes at the names written there, if he will ever come to know the truths they died for.
He's fairly sure that someday, his last day, he will.
What you don't know can kill you.
What you do know can make you willing to die.