An event that occurred after the end of the Third Shinobi World War twelve years before the start of the series. Though many know it to be when Kurama — the Nine-Tails — attacked Konohagakure, only few know what truly occurred and who truly caused the calamity.
The attack of the Nine-Tailed Demon Fox, coincided with Kushina Uzumaki, the Nine-Tails' second jinchūriki, giving birth to Naruto on the night of October 10th. During a female jinchūriki's pregnancy, the energy used to maintain the seal has to be redirected to the growing child in her womb — whether consciously done or not. As a result, the seal used on the beast weakens in direct proportion, and as such special preparations have to be made when a female jinchūriki is about to give birth as the seal may break entirely. Having experienced something similar when the previous jinchūriki of the Nine-Tails, Mito gave birth, the Third Hokage Hiruzen Sarutobi took multiple precautions to have the birthing be performed in secret, on the village's outskirts, surrounded by a barrier, with an elite midwife team consisting of his wife Biwako and Taji along with a few ANBU stationed outside as security. Minato would also accompany them in order to keep the seal in check during the process.
However, the childbirth's location was eventually discovered by Tobi, who learned the information by observing Kakashi at the Konoha Cemetery. Making his way to the site, Tobi killed the ANBU stationed outside and later Biwako and Taji after the child had been born. Holding the newborn Naruto at ransom, he forces Minato to leave the cave via his Flying Thunder God Technique when he sets off the explosive tags he had placed on the newborn's blanket.
Taking advantage of Minato's absence, Tobi forcibly extracted the Nine-Tails from a visibly exhausted Kushina while noting that the Four Symbols Seal also carried the Flying Thunder God Technique seal as a security measure which would enable Minato to teleport to his wife's aid at any time. As the Nine-Tails finally emerged from the seal, Tobi used his Sharingan to gain control of the tailed beast and commands it to destroy Konoha. Kushina who survives the extraction — a feat which Tobi accredits to the famed vitality of the Uzumaki clan — begs Tobi to stop as he orders the Nine-Tails to kill its former host. Minato however arrives in time to save her. The Hokage carried her to his safe-house where he left Kushina with Naruto before heading off to protect Konoha.
Even from a distance, several ninja include a young Itachi Uchiha could sense the disturbance. Tobi successfully sneaked into the village and summoned the Nine-Tails into the village. When Hiruzen felt the Nine-Tails' familiar power and malevolence, he immediately prepared for battle prior to an ANBU arriving to inform him of the Nine-Tails' attack. He ordered every shinobi nearby to prepare to defend the village.
Minato arrived on top of the Hokage Monument, which the Nine-Tails quickly noticed, and attacked him with the Tailed Beast Ball, but Minato teleported the blast away from the village. Before Minato could tell Hiruzen of what had happened at the birthing location, Tobi teleported to Minato and engaged him, but Minato teleported away from the village. Tobi followed suit, and after analysing his abilities, Minato questioned whether the masked man was Madara Uchiha. Giving no definite answer, Tobi attacked Minato as both use their respective space–time techniques to avoid capture. During the struggle, Minato carefully threw his special kunai at Tobi, which the latter allowed to phase through his head, and just as it did, Minato teleported right above his opponent, and landed a timely Rasengan in his back, severely injuring him.
A bewildered and struggling Tobi tried to escape, but Minato, having placed his Flying Thunder God Technique seal on his body as well after his attack, teleported to his location, and placed another seal on Tobi — releasing the Nine-Tails from his control. Proclaiming that Minato truly deserved his title of Hokage for this feat, Tobi teleported away, promising that the Nine-Tails would once again be his.
Despite no longer being under Tobi's control, now motivated by its own malice towards the village responsible for being sealed in the first place, the Nine-Tails continued its rampage of Konoha. However, it was soon pushed outside of Konoha's premises by the village's shinobi. Hiruzen and other shinobi continued their attack on the beast, trying to push it back further. During this time, a young Iruka Umino was protected from the Nine-Tails by his mother as his father tried to help repel the beast. Ignoring his father's orders to leave, Iruka stayed, determined to protect his mother, but another shinobi soon whisks him away from the battle with his parents dying some time after. The younger generation of shinobi were all detained from the battle, despite their ranks which Kurenai Yūhi's father explained was because this was an internal issue, not war. They were moved to the outskirts of the village and protected in a barrier.
Minato returned to the village and was mortified at the destruction the beast had caused. Just as the Nine-Tails was about to fire another Tailed Beast Ball at Konoha, Minato summoned Gamabunta to hold off the Nine-Tails for the moment. Minato then managed to teleport himself and the beast away from the village and to his safe-house, letting the blast detonate without any casualties as he was able to retrieve his wife and child before the ball detonated. Kushina, not one to lay there helpless, manifested chains and a barrier to both restrain and contain the Nine-Tails with the intention of resealing the beast within herself with the process killing them both. However, for various reasons including that death would not stop the Nine-Tails, not wanting his wife to die, or leave Konoha without such a valuable war deterrent, Minato decided seal the beast within his son using the Eight Trigrams Seal.
When Hiruzen and a few other shinobi finally caught up to Minato, they found him, Kushina, and Nine-Tails inside of a barrier which they could not enter and had to watch helplessly from the sidelines. Kushina, initially against their son becoming a jinchūriki, pleas with Minato to live and raise Naruto before being convinced that it is for their son's sake as it is for the village's. While Kushina held down the Nine-Tails with her chakra, Minato uses the Dead Demon Consuming Seal to seal away the Nine-Tails' Yin chakra within himself to weaken the beast — which decreases dramatically in size — before proceeding to beginning the sealing process. Realising Minato's intentions, the Nine-Tails tries to kill Naruto to stop the sealing. However, Naruto's parents stop the Nine-Tails' claw from reaching their son, fatally wounding themselves in the process.
With even less time to spare now, Minato summoned the scroll-toad Gerotora and gave him the key to the seal while Kushina expressed to an infant Naruto how she wanted him to live, to never forget who he was, and how she and Minato loved him. Minato jokingly told Naruto to listen to his talkative mother as he finishes sealing the fox into their son. In the process, as the Nine-Tails curses them for imprisoning it again, Minato also sealed the remainder of his and Kushina's chakra within Naruto to serve as both a final safeguard to prevent the Nine-Tails from totally breaking free, and to assist their son in spirit during any future attempt to take full control of the Nine-Tails' chakra.
Since the resealing of the Nine-Tails happened far from Konoha, only Hiruzen Sarutobi and the two shinobi that accompanied him, bore witness to Minato and Kushina's sacrifice. Hiruzen subverted all information regarding the relationship between Naruto and Minato to protect him, giving Naruto his mother's surname. With the village having suffered heavy casualties during the attack, only a few of the villagers would come to accept Minato's final wish that Naruto be seen as a hero, while out of fear and hatred the others made no distinction between him and the Nine-Tails itself, for the Nine-Tails' actions. This was despite the fact that he was the one containing the beast, preventing it from harming the village.
Because of Minato's untimely death, and having no suitable candidates at hand at the moment, Hiruzen was forced to come out of retirement to retake his position as the current Hokage, despite his age. In later years, as Hiruzen saw the cold treatment of the villagers towards Naruto, he took steps to decree that no mention of Naruto being the Nine-Tails' jinchūriki be spoken with harsh punishment for those who broke the rule. However, many of the villagers continued to ostracise Naruto indirectly, by transferring their harboured feelings of dislike for him onto their children, who too shunned him, resulting in Naruto living a lonely childhood with no friends.
Additionally, as the Uchiha clan's Sharingan was known to be capable of controlling the Nine-Tails, the elders of Konoha suspected them of being responsible for the attack, since the majority of the village was completely unaware of Tobi's presence during this time. Several years later, Sasuke would come to learn this story and that his clan, tired of being marginalised by the Senju, had been planning a coup d'état with his father Fugaku at its helm. This, however, never came to pass due to the clan genocide committed by Itachi Uchiha, which triggered the rise of Sasuke's campaign for vengeance on Konoha after being told by Tobi that it was the elders that had ordered it. Driven and stricken by inconsolable grief-powered rage, Sasuke resolved that the village and its people must pay for enjoying the peace gained for what he had lost, — despite the facts that he would have most likely lost his life along with his clan and many uninvolved innocents if the Uchiha clan rebellion went through — learning that Hiruzen, someone besides a Uchiha, tried his best to help them but was spurned by the clan and his own advisers and being his protector from Danzō, along with the fact that most people are unaware of what really happened. Also, during the attack, Itachi noted that his parents were noticeably absent from the Uchiha compound. Their whereabouts were and what they were doing is currently unknown.
In the anime, it was revealed that the fox had left behind large residual traces of its chakra during the attack. Taking advantage of this for his own agenda, Kazuma collected and sealed the Nine-Tails' chakra within his own son, Sora, turning him into a pseudo-jinchūriki.
Though the Nine-Tails had attacked many settlements regularly throughout history for no apparent reason other than those areas being breeding grounds for the darkest aspects of human nature, Jiraiya and Itachi Uchiha have speculated that Madara Uchiha was behind the attack on Konoha, citing his previous use of the beast in his legendary battle with the First Hokage, Hashirama Senju, while Minato and Kushina were personal witnesses to who they thought was the legendary ninja. Tobi even went as far as to call the attack he orchestrated, a "natural occurrence". However, meeting their son in his subconscious, as they met him on the night of the attack, the chakra apparitions of Naruto's parents warned him be wary of the Akatsuki's "masked man" Tobi. It was only during Naruto's battle with Tobi in the Fourth Shinobi World War that Kurama, having accepting the current jinchūriki as an equal, reveals that Minato sealed it in Naruto to enable his son to stop Tobi once and for all when the time came.