It had started innocuously. Mugen was celebrating his son's second birthday and had invited all his close friends to attend. After liberal amounts of sake, the collective wisdom and impromptu council had decided to celebrate by robbing the Senju. "With no loss of life," Mugen had insisted, not wanting to smear the celebration of his baby boy's birthday with bloodshed.
The more Mugen talked, the more it made sense to everybody present.
Yes, it was the ultimate demonstration of prowess to enter undetected into the heart of the enemy's stronghold, and for the enemy it was the worst humiliation. Doing so without loss of life on either side would show the Uchiha's superior competency, humiliating the Senju further. And they could gloat about it in the next battle, dealing high emotional damage to their psyche, and since it will become known, the Uchiha fame will spread even more.
This should have been tradition, they thought, and if not, they will make it.
Everybody, including Mugen, had forgotten what happens when you mix sharingan, very strong liquors and self hypnosis.
Thus, sharingan spinning, the infiltration plan was unanimously received, and with the aid of highly experimental, done-on-the-fly infiltration genjutsu, they managed to unthinkable and snuck into the Senju stronghold undetected.
There, they fanned out, convening to meet again at the point of infiltration in twenty minutes, time during which they had to take as many valuables as they could.
Tenji had come back first with the Senju Head's sword. Kaoru had come back smiling broadly, swinging a bag full of clinking coins. The rest also trickled back with various trinkets. No alarm had been sounded, for unbeknownst to them, the Senju were also sleeping their own celebration of their Clan Head's youngest son's birthday.
Then, Mugen came back with a sleeping child in his arms.
"What are you doing?" Tenji hissed at him.
Mugen smiled. "Look how cute he is, just my son's age," he said lovingly. "He was all alone, couldn't leave him like that."
Even through the alcohol induced fog, Tenji could see it made no sense. "He was alone in a room in the middle of his home," he hissed, "take him back this instant." But Mugen was drunk on something more powerful than the Lighting daimyo's secret sake stash. "What if he woke up all alone?" He said despondently.
"Then he would have called and someone would have c-" Tenji hissed when he heard three dull thuds.
Kaoru, Suzuka and Yamato looked back at him, at a loss, three unconscious children in their arms. Tenji swore.
"They wanted to sound the alarm," Kaoru hissed. "We had to act quickly."
To prove her wrong, the alarm sounded anyway and they all scrambled back over the wall, sprinting the fields to the forest and towards their own territory.
They split up, signing to meet at a halfway point inside their own territory at the hour of the horse.
Tenji managed to shake off his pursuers and, still under the alcohol's influence (that stuff was strong), did not feel overly panicked at his friends' chances either. His assurance was rewarded when he found everybody already gathered at the rendezvous point. They were huddling together, discussing animatedly.
Tenji strolled over to them. The huddle broke, and Tenji saw the reason. The children were still there.
Mugen made an impassioned argument that they couldn't let the poor children in the wilderness alone, undefended, no matter that the older ones, around twelve or so, just his son's age, were glaring daggers at their captors. If only looks could kill, Tenji thought amused, but at that moment he had not seen anything particularly problematic with taking the kids with them. They had carried them two thirds of the way anyway. Sunken cost fallacy and all that. He was not taking them back. He wanted to go and sleep the hangover off.
But now, after sleeping it off, he realised he had a huge problem. The children were security risks, not to mention the strain on their resources, which were negligible, but they went to a Senju and that was not acceptable.
He called his co-conspirators and laid out the problem to them. Mugen came with both his child and the Senju child in his arms. Tenji sighed and explained the situation while children in Mugen's arms played with his hair beads, rattled their toys and wrestled, laughing loudly.
Snapping out of his daze, in which all the Uchiha present had started shouting encouragements to their clan member to win the wrestling contest, Tenji cleared his throat loudly so he could get their attention once more. "Security risk, strain on resources, we need strategies to recover the risk," he reminded them.
Kaoru shrugged, eyes glinting calculatively. "We could always k-"
"Adopt them!" Mugen said triumphantly.
"What?" Tenji shouted in surprise. "No!"
"We could maim them, make an example out of them," Ugetsu proposed.
"We could exchange them as hostages," Suzuka offered. Tenji nodded. It had an appeal.
"And in the meantime, we can have them put to work," Yamato advised.
"How?" Tenji asked skeptically.
"I believe one of the three is Butsuma, the Senju Head's oldest son. Have Tajima train with him. It will be a great asset for the future, if you do not intend to kill them."
Tenji hummed, unconvinced. But the sunk cost fallacy hung heavy above his head so he decided to give it a try. He could kill them anytime, anyway. In the meantime, they put genjutsu over the whole compound. It was good practice for the younger members of the clan.
On the training field farthest away from their compound, Tajima looked at the prisoners critically. Two boys and one girl, the boys twelve, and the girl thirteen.
He took their blindfolds off. Butsuma started glaring daggers at everybody, which showed he would be a good training partner for his child at least.
Tajima came next to his father to look at the Senju. The other boy stared back insolently. Contact made, hostility established, Tenji cut off Butsuma's ropes, gave him a kunai and a sword, the same weapons as his son, and let them train.
The boys sparred. Or rather, they tried to kill each other, but it was such good training for the real battlefield. He stopped them before they could maim each other permanently, gave pointers to his son and rotated with the other Senju boy while Butsuma rested.
When the other boy, Inari, was too tired to continue, Tenji went for the girl.
She started crying as soon as he let her go, babbling that she was only a maid, that she did not know how to fight. Tenji sighed. There was only one thing one could do with civilians.
"Hikaru!" He shouted. The boy stood at attention. Tenji nodded to the girl.
"Test her, then train her. I want to see results in two weeks."
The girl blanched. Hikaru bowed, slung her over his shoulder and left at a run.
Tenji turned his attention back to the spar and kept Tajima on a roll until the boy was too exhausted to use chakra or even move much.
At this point, he unleashed both Senju on him and an Uchiha for good measure. The Senju, despite throwing him incredulous looks, did attack in good formation.
He stopped them before they could do irreversible damage.
The next two weeks passed in intensive training for his son and Senju, and games for Mugen's son. The two children seemed to have hit it off splendidly, and the young Senju boy was the newest addition at every social gathering between young mothers. Tenji thought he should keep a better eye on him, but the kid had too many eyes on him already, so he let him be. For the time being.
A missive from the Senju finally came, asking for terms of release. He smiled. Despite his initial misgivings, the Senju kids were very helpful for his son. Well, except for the maid. He heard that to explain the use of chakra, Hikaru had punted her off a cliff, then caught her before she could become a splatter on the ground below and informed her he'd do the same every day until she stuk on the cliff face. Apparently, she learned very quickly after that.
Tenji thought he should make Hikaru an instructor. The boy got splendid results.
Deciding to keep the Senju kids around a bit more to have Tajima test some new genjutsu on them, he wrote out a list of ridiculous demands. The Senju understood it was to be the start of a long, drawn out negotiation process, but made the mistake of trying to threaten them back.
Tenji penned a very cold reply, asking them if they were so sure of their position, all the while Tajima and Butsuma squabbled over the right to the last strip of beef teriyaki.
The Senju boy had come to their house in the end, at Tajima's insistence. The second day, during a break, the boys had started talking, and Butsuma had made some disparaging comments on their facilities, remarking that Tajima definitely did not have his room arranged according to the Five Governing Principles. Tajima had been outraged because he so had his room designed correctly, and moreover he had an absolutely unrivaled collection of kunai from all the nations.
"For the pride of the Uchiha, I need to show that Senju our superiority," Tajima had told his father, and Tenji agreed. They could always kill Butsuma later if problems arose. Tajima could also benefit from having to keep under surveillance a prisoner at all times. His room was big enough for ten kids to sleep in, anyways.
"Just let the guest have it, Tajima," his daughter said, bringing him back to the present.
Tajima looked around theatrically. "Which guest?" He asked. "There is no guest. That one," he said pointing with his chopsticks rudely, "is a prisoner."
"You are being rude!" Sayuri snapped. "He has better manners than you. Do you want a second portion, Butsuma kun?"
"Butsuma kun" politely agreed, complimenting her cooking skills. Tenji suggested Butsuma train with Sayuri the next day and chuckled when the boy seemed uncomfortable with the idea.
After Sayuri curb stomped him, Butsuma seemed to revise his opinion, unfortunately using her to annoy Tajima by comparing him disfavorably with his older sister. Tenji let it be, as it built character.
Finally, after three months he got to a satisfactory result regarding the ransom. The prisoners were all eager to go back, all that is, except for the youngest kid who cried and clutched at Mugen's clothes as if he was being abandoned. Satoru, Mugen's boy, refused to let him go. Tenji was forced to threaten Mugen with the loss of his own family if he kept that up.
In the end, they managed to foist them off into their clan's hands and get their reward and all was well. The conflicts petered out for a while, as both clans suddenly got so many missions they did not have force to spare for their own battlefield. They were also not hired by opposing factions, which was a nice change of pace.
The Senju never managed the same feat of infiltrating their stronghold, a fact he gloated about to any Senju he came across during missions.
All was well, until, four years later, Hikaru and the Senju girl eloped and disappeared without trace.
The simmering conflict between the clans exploded anew.