Prologue
For as long as anyone can remember there has been clans. What clan you're from decides what your abilities are, and who you're friends with.
The Uchiha and Senju clan have been enemies for centuries, almost always at war with each other. There was no way around it, that's just how they were. At least until Hashirama and Madara. Hashirama was the heika of the Senju clan while Madara was the heika of the Uchiha clan, they met by chance one day and quickly became friends even though they had no idea what clan the other was from. They grew closer every day, talking about whatever, sparing, or just plain goofing off. They promised that if they ever met on the battlefield they would not kill each other, or the other's remaining brother.
One day Hashirama's father found out about their little meet ups, and decided to take care of the little Uchiha himself. He and a group of shinobi followed Hashirama and they attacked Madara. He felt betrayed because he thought Hashirama purposefully led them there and swore to wipe out the Senju clan as he left, not without shedding unseen tears for the end of his and Hashirama's friendship. Years passed and they finally saw each other again, but it was on the battlefield. They fought for a long time, after all their clans were enemies. However, one particular time they fought Hashirama managed to win, but he refused to kill Madara, instead he talked about their childhood and offered an alliance. The Uchiha and Senju becoming allies was one of the most shocking things to ever happen.
The Uzumaki clan was closely related and allied to the Senju clan, so the alliance extended to them as well.
The alliance allowed the friendship between Mikoto and Kushina to exist. Mikoto was an average Uchiha, while Kushina was the heika of the Uzumaki. They met when Kushina's parents where visiting the Uchiha royal family, they quickly became friends and hung out the whole day, and when Kushina had to go back home Mikoto was there to see her off. She was a little surprised that her new friend was the Uzumaki Heika but swore she would treat her no different from her other friends.
They wrote to each other frequently, and met in person whenever they could. They were together through thick and thin, bad and good. Mikoto had a child first, a son named Itachi, Kushina and her husband Minato came to visit and congratulate Mikoto and her husband Fugaku, the current Uchiha Tonno. Years later they both found out they were pregnant and decided to raise their children together as much as possible. Kushina named her son Naruto and Mikoto named hers Sasuke.
Sasuke and Naruto were best friends and did everything that they could together. Whenever they found out they were visiting the other they got really excited and couldn't wait to get there. They were friendly rivals, always trying to outdo the other, but they cared very deeply about each other. But when the boys were 7 years old the Uchiha and Uzumaki lost Kushina and Fugaku in battle. Both clans mourned the death of their tonnos, but Sasuke and Naruto mourned the death of their parents. Itachi immediately became the tonno of the Uchiha, much to the protest of Mikoto. She said he should enjoy his childhood while he had it and that she'd lead until he was older, but he insisted. Saying he was basically an adult, and could take this on himself.
Minato became the tonno of the Uzumaki because Naruto was way too young to lead, and would have to give up his position when Naruto was old enough. Neither of them was the same. Naruto, who was now the kyuubi jinchuuriki, became a huge prankster and Sasuke, who watched his father die, became more aggressive. However, they remained friends, even though they saw each other less often.
The two of them grew up and continued their training, they had already learned quite a lot about being a shinobi by the time they were 6, so they continued to train in ninjutsu, taijutsu, genjutsu, fuinjutsu, bukijutsu, kenjutsu, and their kekkei genkai. While they were better at some of them and worse at others, they pushed on. Naruto's goal was to become strong enough so that no one precious to him would die, while Sasuke wanted to get stronger to kill the ones who killed his father and Naruto's mother. By the time they were 12 they were one of the most powerful shinobi in their respective clans. However, fate had a plan for these two.