Warning: This might be spoiler-y if you are still watching the Naruto anime.
In the final arc of Naruto, when things are going crazy and Naruto and Sasuke nearly die in their fight against Madara, they go to a strange place between life and death, and there they meet Hagoromo Otsutsuki, who is better known as the Sage of the Six Paths
The Sage of the Six Paths tells them the story of his life, of his mother and the divine tree and chakra and all of ninshu. He speaks a lot about his two sons, Indra and Asura.
Indra was the first son of Hagoromo, and inherited his strong genes. From a young age, he demonstrated a great aptitude towards ninshū and anything else he applied his mind to. Due to his prowess, he led a solitary life as he felt no need to depend on others.
As a child, Asura lived in the shadow of his genius older brother Indra Ōtsutsuki. Having displayed no natural talent himself, Asura came to realise the value of others and the power of the friends and allies he made. Eventually, he awakened his own power which equalled Indra's own.
When their father was on his deathbed, Asura Ōtsutsuki, Indra's younger brother, was chosen to carry on his dream of establishing peace throughout the world. Indra, believing that he should have been chosen instead as it was his birthright, was manipulated by Black Zetsu into fighting Asura.
In doing so, Indra was the first to develop ninjutsu from his father's ninshū. Both brothers died fighting over the title of the successor of Ninshū, but not before having offspring who would continue this feud for centuries to come.
Indra's eventual descendants — the Uchiha clan — would feud with Asura's eventual descendants — the Senju clan. Neither clan knew why or how their hatred of the other came to be.
The brothers' feud only came to an end with their reincarnations in Naruto and Sasuke, who battled calamitously to resolve the conflict in ideologies. (via)