The founding of Konohagakure came after many decades of ceaseless war, during which time even children were not spared the horrors of the battlefield. Because of this, the life expectancy of shinobi at this time was only 30 years. Sarutobi Hideyoshi was one of these child soldiers, and he dutifully hardened his heart and took on the mantle of heir the Clan after his older brother was killed by enemy shinobi. Until one day, when he was a young man, and he was forced to kill a child no more than 10 years old. He had encountered this child at the end of a long battle, and in his last moments Hideyoshi saw the face of his brother, of himself, of all of the children who had been victimized by this pointless war. He decided then and there that he would put a stop to the violence, no matter what it took.
Unfortunately, under his father's leadership, the Clan refused to abide by Hideyoshi's wishes, and so he left on a pilgrimage to Hell's Valley, the place where the monkeys lived. Long time friends of the Sarutobi Clan, they invited him to stay and train with them until he figured out what he should do. Thus he began his 20 year long training retreat, and stayed there until news came of his father's death, and the Sarutobi Clan left without a leader. It was then that he returned to his clan, to be their guide in this time of confusion and strife. He would lead them into a new age. They withdrew as much as possible from the wars, refusing to accept any more contracts from greedy daimyos, and retreated into the forests to create their own wealth and live off the land, as the wise monkeys do.
It took many years of defending against attackers, but the Sarutobi's patient resilience and commitment to peace eventually drew the attention of the other clans, and he was able to enter into talks and conferences with them to explain his standpoint. Among the first he won over to his side were the Yamanaka, Nara, and Akimichi clans, and to this day they retain a unique bond of shared ideology. In order to solidify their cause and win the support of the Feudal Lords, however, he still needed the support of one more clan.
The powerful Uchiha were not easily won over, and the Clan Head of the Uchiha demanded victory in singular combat in exchange for his support. HIdeyoshi agreed, and the two did battle. The fight lasted for over three hours before Hideyoshi was able to pull off a narrow victory, after which the Uchiha Clan acknowledged his plea and agreed to support him in his movement.
With the clans he brought to his side, Hideyoshi was able to convince the Land of Fire to permit him to build the first hidden shinobi village, right there in the forests where the Sarutobi had settled. Under his leadership, Hideyoshi hoped that the clans would at last know peace, though he knows that for his dream to be fully realized, there was a lot more work yet to be done. For peace is not easily won, and it is not easily kept, but it is always worth fighting for.