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 and has been rarely shown in the series outside Kakashi Gaiden. 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. These two villages finally made peace after the Hyūga Affair. 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. Despite their attempts, the shinobi were slaughtered by a grief-fueled Obito Uchiha who, unbeknownst to his team-mates, had actually survived the landslide that had seemingly killed him. 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. In the anime, both the Third Tsuchikage and Third Hokage had grown weary of sacrificing more lives to an ongoing stalemate and finalised a peace treaty over the objection of military hard-liners in their respective villages. However, Ōnoki was more relunctant to accept the peace treaty, only accepting it after Minato single-handedly stopped an invasion consisting of a thousand Iwa shinobi.

Effects
Minato Namikaze was a key factor in Konoha's victory in the war, gaining the moniker of and being named Konoha's Fourth Hokage (四代目火影, Yondaime Hokage; Literally meaning "Fourth Fire Shadow") at the war's conclusion. Kakashi Hatake, Minato's student, gained a Sharingan in his left eye from his team-mate, Obito Uchiha during the mission to destroy the Kannabi Bridge. He would go on to become renown as. Obito himself would be saved by Madara Uchiha. After witnessing the death of his beloved Rin, Obito would lose all hope in the shinobi world and awaken his Mangekyō Sharingan. Obito would later take on Madara's identity after Madara's death and became dedicated to fulfilling the Eye of the Moon Plan and would eventually spark the Fourth Shinobi World War.

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 shape the rest of his life.

Sasori earned fame as, due to his ingenious puppets dyeing the sand red with his enemies' blood. At some point however, Sasori left the village. Years later, he would secretly kill the Third Kazekage and turn him into one of his human puppets, leading to the appointment of the Fourth Kazekage after a long but futile search for the Third by Sunagakure. The Battle of Kikyō Pass, where Kabuto Yakushi was said to have been found orphaned (although this was later revealed to be a cover), took place in this war.

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, Nagato responded by summoning the Demonic Statue of the Outer Path which 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.

In Kirigakure, Ao gained a Byakugan as a spoil of war from a Hyūga clan member he defeated. This allowed him to see through the hypnosis that controlled the Fourth Mizukage.

Furthermore, Hiruzen decided to step down as Hokage, with Minato succeeding his rule instead of listening to Danzō and promoting Orochimaru.

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.