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.
As with all wars, the Second Shinobi World War brought forth many heroes and took even more lives. It was during this war that the legendary "Sannin" rose to fame and received their nickname from Hanzo 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 is also likely that this war was 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 Kato, leaving her with an extreme case of hæmophobia. It was also this 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.