Fourth Shinobi World War

The beginning of the was declared by Madara Uchiha when the five Kage refused to give him the Eight-Tailed Giant Ox and the Nine-Tailed Demon Fox for his Moon's Eye Plan. Madara has stated that he will use the seven tailed beasts already captured by the Akatsuki in combating the ninja world. The Fifth Kazekage, the Fifth Mizukage, the Fourth Raikage, the Third Tsuchikage, and General Mifune have agreed to form a Shinobi Alliance, pooling their resources against Madara. The daimyo of the Five Great Shinobi Countries and the Fifth Hokage later join the alliance as well.

Preparations for War
Thus far both sides are gathering their strength and collecting information about the other side in preparation for the upcoming hostilities. Kisame continues to gather intelligence on Kumogakure's military strength and on Killer Bee traveling with him everywhere while hidden inside Samehada. The team of Anko Mitarashi tracks Kabuto Yakushi, which led them to Madara Uchiha's lair. Other Shinobi Alliance teams have fanned out to collect other information about Akatsuki and their hideouts.

Kabuto has formed an alliance with Madara, showcasing his use of the Summoning: Impure World Resurrection technique, increasing the forces of Akatsuki with the resurrection of dead members of the organization. The Five Kage have decided to hide the two remaining jinchūriki, Naruto and Killer Bee in a Land of Lightning Island. Hiding Killer Bee and Naruto had the unintended result of allowing Naruto to learn to control the Nine-Tailed Demon Fox's chakra, much like the way Killer Bee has of the Eight-Tailed Giant Ox. Having been successful in his training, Naruto has exhibited increased speed and strength when utilizing the controlled chakra of the Nine Tails.

Trivia

 * Itachi Uchiha prevented the outbreak of a Fourth Ninja War by carrying out the Uchiha Clan Massacre.
 * The Fourth Ninja War was also declared by Hiruko in the third Naruto Shippūden Movie, and would have pushed through if not for the efforts of select Konoha shinobi.