Naruto featured one of the best story-telling in Shounen, and in media, about how it all started when two young children, Hashirama Senju and Madara Uchiha, dreamed of ending the war. Later, this dream was only realized after sacrifice, but the two founders couldn't stay together despite smooth sailing.
When these two were younger, their clans were driven by war and their hate against one another. Both of them had siblings whom they loved, and dying on the battlefield was an honor, so no one stopped children from dying.
One of these siblings was Itama Senju, the brother of Hashirama and Tobirama. Itama died a very painful death when some Uchiha clan members cornered him. But was his death necessary to continue the story of Naruto?
Naruto: Discovering whether or not the death of Itama Senju was necessary
Hashirama Senju and every other Hokage that had died were reincarnated during the Fourth Great Ninja War to fight the reincarnated Madara Uchiha. After being reincarnated, Hashirama dwelled back into his past as he thought of how he and Madara once laid the foundation of Hidden Leaf Village and is now returning to destroy it.
From Naruto chapter 621 to chapter 626, Hashirama's origins were revealed, on how he met Madara and established the Hidden Leaf Village. When Hashirama was young, he lived in a world at war, and not even children were safe from fighting.
One day, he saw his brother Kawarama Senju get buried before him and realized that the world needed to change. Earlier, he met Madara as they parted ways after exchanging names.
He bluntly put everything out in front of his father, who criticized him, saying that this was the world's order and dying on the battlefield was nothing but an honor for a ninja. Tobirama and Itama took him aside and said they agreed with everything he said but could not do anything about it.
Itama was very kind-hearted and mourned the death of his brother Kawarama. Just like Hashirama, he also wanted the war to end. Unfortunately, Iwata was surrounded by some Uchiha clan one day, and he died at their hands.
This depressed Hashirama and his personality changed as he stopped smiling. Madara noticed this during their next meeting and inquired about what had happened. Hashirama opened up, and this led to their friendship growing close.
So, Itama Senji's death could be considered crucial to the plot of Naruto. Itama's death led to Hashirama losing all hope and falling into depression. This made the latter close to Madara, and they told each other their dream of ending the war after signing a treaty.
Although they were met with problems until both clans signed a treaty, Itama's death triggered the relationship between Madara and Hashirama. After signing the treaty, these two figures founded Hidden Leaf Village in Naruto.
Unfortunately, the good times didn't last long as Madara declared war against Hashirama and left the village. During their second battle to death, Hashirama took no chance and stated that he would kill anyone who threatened the existence of his people, be it friend or foe.