Second Shinobi World War

The was the second of four great wars involving the majority of the shinobi villages and countries. Not much is known about the politics of this era, but there were known conflicts between Konohagakure, led by the Third Hokage, Sunagakure, likely led by the Third Kazekage, Amegakure, led by Hanzō of the Salamander, and Iwagakure, presumably led by the Third Tsuchikage. The majority of the fighting seems to have taken place in minor countries like Amegakure, leaving them devastated. The eventual outcome of the war seems to have been in favor of Konoha. Nagato referred to it as a war Konoha started.

Background
The First Shinobi World War ended with an armistice treaty, but also left the Five Great Shinobi Countries seriously wounded. The treaty led to a period of peace, but after about twenty years, the economic disparity between the countries had become a serious problem. The nations began to form factions and, under the pretext of expending fair rights, the countries started using military force to expand their territories. Thus, the gruesome Second Shinobi World War started.

Effects
As with all wars, the Second Shinobi World War brought forth many heroes and took even more lives. It was during this war the legendary Sannin rose to fame and received their nickname from Hanzō during a skirmish with Amegakure. Tsunade in particular became famous as the world's greatest medical-nin while successfully countering all of Chiyo's poisons. It was also likely the war where Sakumo Hatake made his name as "Konoha's White Fang".

On the other side, it was the war where Tsunade lost her younger brother Nawaki and her lover Dan, leaving her with an extreme case of hæmophobia. It was also the war that took the lives of Sasori's parents, an event that eventually led him to desert his village and join Akatsuki. The Ame Orphans lost their parents during this war, leading Jiraiya to take care of them for a while, claiming it was the least he could do. One of the orphans, Nagato, would come to desire an end to war because of his experiences from this war, a purpose he used Akatsuki for.

Trivia

 * In the anime, the Sannin's actions during this war are incorrectly stated to have occurred during the Third Shinobi World War.
 * It was during this second war that the Land of the Sky emerged to challenge the Five Shinobi Nations, only to be destroyed, as revealed in the second Naruto: Shippūden movie, in which the Sky ninja's descendants re-emerged to attack.