The beginning of the Fourth Great Shinobi World War is declared by Madara Uchiha when the five Kage refuse to give him the Eight-Tailed Giant Ox and the Nine-Tailed Demon Fox. Madara has stated that he will use the seven tailed beasts already captured by Akatsuki in combating the ninja world. The Fifth Mizukage, the Fifth Kazekage, the current Raikage, the current Tsuchikage, and Mifune have agreed to form a Shinobi Alliance, pooling their resources against Madara. They intend to invite the ninja of Konohagakure into the alliance, but they do not trust the Sixth Hokage.
Years earlier, Itachi Uchiha had prevented the outbreak of a Fourth Ninja War by carrying out the Uchiha Clan Massacre.
A Fourth Ninja War was also declared by Hiruko (missing-nin) during the third Naruto Shippūden Movie, but that never happened.