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 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 island in the Land of Lightning. 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.

Both the Allied Shinobi Forces and Akatsuki have amassed considerable forces for this war, 80,000 shinobi and samurai total make up the Alliance and 100,000 Zetsu clones and at least 24 warriors of note revived by Kabuto make up Madara's army. The first large scale battle between the two forces seems to be on the horizon, and Madara had dispatched Kabuto to retrieve the Nine-Tails and Eight-Tails, with Ōnoki going to reinforce the forces on the Island Turtle and was able to stop Kabuto, but Kabuto still made it out with Yamato, intending on strengthing the Zetsus with Yamato's powers.

Joint Shinobi Army
The Joint Shinobi Army created by the Five Great Nations consists of five main divisions, with Gaara of the Sand as the Commanding General. The 1st Division consists of midrange fighters and is led by Darui. The 2nd Division is made up of Close Range Fighters and is led by Kitsuchi. The 3rd Division is under the command of Hatake Kakashi, and features both close and midrange fighters. The 4th Division contains only long range fighters under the control of Gaara, who is also the commanding general of the entire army. Shikamaru is Gaara's right hand man and proxy general of the 4th division as well. Finally, the 5th division was created for the purpose of special operations, and features ninjas with unique skills. Mifune is the leader of these special operatives. There is also four special company formed for the war. The Medical Logistics Company led by Shizune comprises of medical ninja, the Ambush Company led by Kankurō, has the task to ambush the enemy forces. The second is the Sensor Company with Ao as captain, and finally the Intelligence Company headed by Inoichi Yamanaka.

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 potential 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.