Second Shinobi World War

The was the second of three 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 Salamander Hanzō, 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. It was also during this conflict that the Sky Country 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.

Background
The First Great 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 Great Shinobi World War started.

Effects
As with all wars, the Second Great 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 possibly 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 the war, so Jiraiya took care of them for a while, claiming it was the least he could do. One of the orphans, Nagato, would later meet with Madara and form Akatsuki with him because of his experiences during this war. He would act as the leader of this organisation, reporting only to Madara.

Trivia

 * In the anime, the Sannin's actions during this war are incorrectly stated to have occurred during the Third Great Shinobi World War.