Third Shinobi World War

The has been rarely shown in the series outside the Kakashi Gaiden. Because of a decline in national power, the reign of the Five Great Countries was crumbling. Along their borders, skirmishes with smaller nations broke out all the time. The prolonged war gradually spread its fires far and wide, until at last it developed into the Third Shinobi World War. This war 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 were thrown unto the battlefield, losing their short lives.

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. Sunagakure also participated in the war, as it was during this war that Sasori earned fame as, due to his ingenious puppets dying the sand red with his enemies' blood. Kumogakure was also seemingly a part of this was, with at least Konohagakure clashing with shinobi from this village.

Minato Namikaze became a hero during the war and gained fame as. Kakashi Hatake, who was promoted to jōnin and gained his Sharingan from Obito Uchiha during the mission to destroy the Kannabi Bridge, also gained fame as. Itachi Uchiha was traumatised by the violence of the war and grew to seek peace at any cost because of it; The rest of his life would be greatly affected by this experience. 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 fought the rule of Hanzō. Hanzō called in the assistance of Danzō and his Root ANBU, as well as held Konan hostage but failed to suppress the group. The confrontation between the two sides lead to a slaughter of all Amegakure and Root shinobi present by Nagato, with the exception of Hanzō who fled and to the death of Yahiko.

In the anime in Part II, Hiruzen Sarutobi made a peace treaty with the Tsuchigumo clan sometime during the war.

Trivia

 * It was claimed in Naruto Shippūden 3: Inheritors of the Will of Fire that during this war, the missing-nin Hiruko was actively working to finish his Chimera Technique and have 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 4: The Lost Tower that Rōran was destroyed during this war.