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 and skirmishes among smaller nations broke out on daily basis that set up the stage for 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 excepting a great power like Konohagakure, very young children were thrown unto the battlefield and into early graves. Being among those sent to war, Itachi Uchiha was traumatised by the carnage and vowed to achieve peace at any cost.

When Iwagakure infiltrated Kusagakure to the border of the Land of Fire, Kakashi Hatake was among the selected ninja sent to destroy Kannabi Bridge. Though Kakashi turned the war in Konoha's favour, it resulted in the death of his team-mate Obito Uchiha during the mission as Kakashi received his friend's Sharingan eye and gained fame soon after as. It was also revealed at the time that Tobi also acquired a Sharingan in the aftermath of the Kannabi Bridge's destruction.

The Battle of Kikyō Pass, where Kabuto Yakushi was said to have been found orphaned, took place in this war. Minato Namikaze became a hero during the war and received his nickname,. 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.

In Amegakure, Yahiko, Nagato, Konan, and many other shinobi formed a liberation 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 the death of Yahiko followed by slaughter of all Amegakure and Root shinobi present by Nagato, with the exception of Hanzō who fled.

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 Hiruzen eventually 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.