Fourth Shinobi World War

The beginning of the was declared by Madara Uchiha when the five Kages refused to give him the Eight-Tailed Giant Ox and the Nine-Tailed Demon Fox for his Eye of the Moon 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.

Despite being captured in the end and committing suicide to keep the information he held secret, Kisame was able to finish Akatsuki's preparations for the War by sending a summon shark with a scroll to his comrades.

Battle for the Rinnegan
What can be considered the first true battle of the war was Konan vs Madara Uchiha over the location of Nagato's corpse. Konan had prepared for the battle, having integrated explosive tags into her body as well as placing 600 billion of them into her Paper Person of God Technique. Madara underestimated her prowess, losing an arm and part of his mask while she took advantage of his 5-minutes limited of intangibility with 10 full minutes of explosion. When Konan thought she had won, Madara surprised her with his Izanagi to escape her technique, and he used a genjutsu to get the location of Nagato from her, then killing Konan and retrieving the Rinnegan.

Trivia

 * Itachi Uchiha prevented an earlier 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.