The First Shinobi World War (第一次忍界大戦, Daiichiji Ninkai Taisen), or simply the Shinobi World War (忍界大戦, Ninkai Taisen), was the first of the great wars that involved the majority of shinobi villages and countries. There are few reliable surviving records of this war, but the experiences of this war led to the birth of the tactics that still form the cornerstone of warfare. It ended with an armistice treaty, but only after all of the Five Great Nations had been seriously damaged.
The war started not long after the system of one shinobi village per country was established by the founding of Konohagakure about 80 years before the current storyline. To try and maintain a balance of power, the First Hokage divided the tailed beasts among the other five villages when he began his peace talks with them. However, this was not enough to prevent the war, and may have actually increased hostilities.
The Second Hokage died during this war, while fighting against Kumogakure's Kinkaku Force, sacrificing himself as a decoy in order for his subordinates to escape. Before doing so he selected Hiruzen Sarutobi for the position of Third Hokage.