Third Shinobi World War

The was the third of the four shinobi wars that involved the majority of the shinobi villages. It took place more than ten years prior to the beginning of the series. Because of a decline in national power, the reign of the Five Great Shinobi Countries was crumbling. Along their borders, skirmishes with smaller nations broke out all the time. The prolonged conflicts gradually spread their flames far and wide, until at last it developed into an all-out war. The conflict turned into an unprecedented war of attrition, tormenting all nations with a shortage of war potential. Not even excluding a great power like Konoha, very young children, some of whom were barely out of the Academy were thrown onto the battlefield, eventually losing their young lives during the war.

War
Konohagakure and Iwagakure fought across Kusagakure after the latter had infiltrated it in order to lay siege to the border of the Land of Fire. It was not until the destruction of the Kannabi Bridge that the war began turning in Konoha's favour. Iwagakure was also known to have been fighting Kumogakure with various skirmishes between the two villages. Sunagakure also participated in the war. Kumogakure was also seemingly a part of this war, with at least Konohagakure clashing with shinobi from this village, specifically Minato and A. This is also the war in which the Third Raikage died and the Fourth Raikage was appointed. Kirigakure also partook in the war against Konohagakure and made Rin Nohara into the jinchūriki of the Three-Tails. Rin chose to die by Kakashi's hands, rather than be used as a tool to destroy her own village. The Kiri shinobi were adamant in retrieving Rin's body and not allowing it to fall into enemy hands. Rin achieved this by letting herself be stabbed by Kakashi's Chidori which not only stopped the Three-Tails from being released but also resulted in Rin's death. Grief-stricken Obito Uchiha, who unbeknownst to his teammates, had survived and was a witness to Rin's murder, massacred the Kiri shinobi by himself, despite their best attempts. Some time after this, Madara Uchiha would pass away, but not before passing his plans onto Obito to fulfil until the time of his resurrection. Ao from Kirigakure also defeated a member of the Hyūga clan and took the member's Byakugan for his own right eye socket. The Battle of Kikyō Pass took place in this war.

The war that swallowed up shinobi from every land came to an end after an armistice agreement between Konoha and Iwagakure, which were the principal aggressors of the war, was concluded. In the anime, the Tsuchikage was more reluctant to accept the peace treaty, only accepting it after Minato single-handedly stopped an invasion consisting of a thousand Iwa shinobi.

Effects
In Konoha, although the war had been proceeding favourably for the village, the Third Hokage established a policy of reconciliation to bring an end to the fighting with an unprecedented offer to not seek reparations from Iwa. Advocates for the war opposed Hiruzen's seemingly weak decision, and to keep dissatisfaction in the village in check, he decided to step down as Hokage. The Konoha shinobi that received the most intention for their war efforts in the village were the Sannin and the surviving members of Team Minato. Out of the two teams, Minato who was the key factor in the war that gained the moniker of alongside Orochimaru, were the only two candidates village officials put forward to succeed Hiruzen. Ultimately, it was Minato who became the Fourth Hokage.

Between Fugaku Uchiha not receiving praise for his work during the war alongside his name not being put forth to become Hokage, members of the Uchiha clan came to despise village officials. Itachi Uchiha was traumatised by the violence of the war and grew to seek peace at any cost because of it: an experience that would lead to the Uchiha Clan Downfall and prevent his clan triggering the Fourth Shinobi World War. However, Obito Uchiha who had been saved from death by Madara Uchiha during the war eventually caused the next world war, after witnessing Rin Nohara die and taking on his saviours identity.

In Sunagakure, Sasori earned fame as, due to his ingenious puppets dyeing the sand red with his enemies' blood. In Kirigakure, Ao gained a Byakugan from a Hyūga clan member he killed during the war, which in term lead him to be known as. In Konoha, Kakashi Hatake eventually gained fame throughout the shinobi world as, after he gained a Sharingan from a dying Obito during their mission to destroy the Kannabi Bridge.

In Amegakure, Yahiko, Nagato, Konan, and many other shinobi formed a group that eventually fought the rule of Hanzō. Initially, they were merely preaching peace for Amegakure. They were supported and even invited to negotiate peace by Hanzō himself. In the anime, they unknowingly disrupted Danzō's attempt to disrupt a ceasefire between Konohagakure and Iwagakure. After Danzō used his men's corpses to frame Akatsuki of murdering his village's shinobi, Hanzō came to fear that they were power hungry and would usurp his power. He betrayed them and called in the assistance of Danzō and his Root Anbu while holding Konan hostage. The confrontation between the two sides led to Yahiko committing suicide, choosing to do so as Yahiko did not want Nagato living with the memory of killing Yahiko. Left devastated by Yahiko's death, Nagato responded by summoning the Demonic Statue of the Outer Path which subsequently slaughtered all of the Amegakure and Root shinobi who had been instructed to kill Nagato. Nagato also attempted to kill Hanzō who fled. This incident failed to suppress the group and led to a civil war with Nagato becoming the new leader of Akatsuki. The resulting civil war split Amegakure and ultimately resulted in the complete and utter annihilation of Hanzō's faction with Hanzō himself eventually dying at the hands of Pain/Yahiko.

Trivia

 * It was claimed in Naruto Shippūden the Movie: The Will of Fire that during this war, the missing-nin Hiruko was actively working to finish his Chimera Technique and had tracked Kakashi in order to brand him with his Puppet Curse. He claimed that, eventually, the Third Hokage ordered his assassination to be carried out by his former friends, the Sannin.
 * It was claimed in Naruto Shippūden the Movie: The Lost Tower that Rōran was destroyed during this war.
 * In Six-Tails Unleashed, the Third Hokage made a peace treaty with the Tsuchigumo clan sometime during the war.
 * In the anime, Danzō was shown to be a driving force in this war, having members of Root transform into Iwa-nin and frame the village to cause war between the two villages.