The First Great Ninja War, was the first of the great wars that involved the majority of the ninja villages and countries. There are reliable surviving records of this war led to the birth of the tactics that still form the cornerstone of warfare.
The war started not long after the system of one shinobi village per country was established by the founding of Konohagakure. 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 after the end of the war, while fightning against Kumogakure's Kinkaku Force, sacrificing himself as a decoy in order for his suborinates to escape. Before doing so he selected Hizuren Sarutobi for the position of Third Hokage.
This war was ended by the Founding Second Hokage, Founding Second Mizukage, Founding Second Raikage, Founding Second Kazekage, and the Founding Second Tsuchikage and with an armistice treaty, but only after all of the Five Great Nations has been seriously damaged.