The Allied Shinobi Forces is a coalition between the collective military might of the hidden villages of the shinobi nations of the Lands of Lightning, Water, Earth, Wind, and Fire, and the lone samurai nation of the Land of Iron.
The purpose of this alliance is to stand up together against the threat of the Akatsuki organisation and the tailed beasts in the Fourth Shinobi World War. Because of the deceitful actions of the Sixth Hokage Candidate Danzō Shimura, the other Kage asked Kakashi Hatake about Konohagakure's entrance into the alliance as they deemed him the most suitable candidate to assume the office. However, before Kakashi was made Hokage, Tsunade woke up from her coma, and resumed her post. After the summit, the five daimyō agreed to give power to their Hidden Villages, making the Shinobi Allied Forces official.
The coalition was formed after Tobi's declaration of war after the five Kage and the head of the Land of Iron, Mifune, unanimously refused to hand over Killer Bee and Naruto Uzumaki, the jinchūriki of the Eight-Tails and the Nine-Tailed Demon Fox respectively at the Kage Summit. When Tobi departed, the Kage and Mifune discussed their plans on dealing with Tobi and the tailed beasts and later Mifune suggested that the Fourth Raikage be the Supreme Leader (大権, Taiken) of the alliance to which the other Kage agreed.
The coalition's primary objective was to defend the Eight-Tails and Nine-Tails' jinchūriki from Akatsuki. The two jinchūriki, once caught, will complete Tobi's Eye of the Moon Plan. Therefore, the coalition planned to first find the two jinchūriki before Akatsuki finds them and keep them safe and away from the battlefield if necessary. Beyond that, their duty is to put up a united front against Akatsuki's army.
At the suggestion of the Fifth Mizukage, the five Kage agreed to merge their intelligence unit into a single, separate organization, to optimize the processing and sharing of new information. This is a significant development with even the Third Tsuchikage agreeing without hesitation, despite his past prejudices and the potential damage such an organisation could cause for his own intelligence forces if they should win the war; which demonstrates how significantly they are treating Tobi as a threat.
To show their unification, Mifune has designed a forehead protector with the kanji "Shinobi" (忍) on it. All members of the alliance wear this in place of their original headbands, to show that everyone is truly united under one banner, as opposed to being separated into five different villages. Most members of the alliance also took to wearing the basic uniform of their village, albeit with the new forehead protector to unify them.
The first internal dissent came from the Fifth Hokage who originally disagreed with how the jinchūriki were to be hidden instead of being their main forces of war, and due to her past defence of Naruto she probably only relented due to the need for a united front. As the forces gathered, there was some tension between the ninja of different villages that led to arguments and conflict due to past experiences, until Gaara made his speech that got them to cooperate more willingly.
Externally, the Water Daimyō expressed extreme reluctance to agreeing to this alliance since it would give the shinobi even greater power, despite its necessity to stand up to Akatsuki's terrorism.
Later on, when Naruto Uzumaki and Killer B escaped from confinement to participate in the war, A decided it was best to kill Naruto for not submitting to his orders. Because A's decision was not sanctioned by the rest of the Kage and the high losses on the Allied Forces side, Tsunade decides to confront A and allow Naruto's involvement. After a long fight, A admits defeat to Naruto and allows him and B to pass.
The Allied Shinobi Forces total army has around 80,000 people, among them shinobi and samurai alike. By the end of first day of battles, the army lost half of its strength. The alliance base of operations is located south-east of Kumogakure, on the east coast of the Land of Lightning. In order to prepare for battle, the ninja have been divided up into nine divisions pooling together ninja with similar abilities. The Great Battle Regiment (戦闘大連隊, Sentō Dairentai) is divided into five divisions that are each lead by a commander (部隊長, butaichō), with Gaara, the Fifth Kazekage, acting as the Regimental Commander (連隊長, Rentaichō) of the entire regiment. Similarly there are four supporting divisions for the army each headed by a captain (隊長, taichō), as well as special teams within the great regiments which are specialized in the usage of certain chakra natures. Furthermore, within each division there are subsets of teams such as the: Sealing Team (封印班, Fūinhan) to help seal those reincarnated through the Summoning: Impure World Reincarnation technique and the Communications Team (連絡班, Renrakuhan) whose job is to stay in contact with headquarters as well as other divisions in order to receive and pass on intelligence quickly and efficiently. Because of the fact that they could be used as high priority ransom pieces in the war, the daimyō were also assigned to a protection squad, which is led by Mei Terumī, the Fifth Mizukage. The anime shows that the army also contains the special Water Attack Raid Unit, which is seemingly specialised in aquatic combat.
At the headquarters of the Alliance, the Kage are seated at the front of the room at a table filled with the intel that they have garnered from reports as well as intelligence that was previously gathered. The heads of the Intelligence and Sensor Division: Inoichi and Ao respectively, are also stationed at the headquarters along with other members of their division. Inoichi along with two other shinobi from his division are hooked up to a large-scale version of the Chakra Transmission Communication Device that is on the direct opposite side of the room which allows them the receive and transmit intelligence to relevant parties. He also uses the machine in conjunction with his own telepathic abilities to allow others to communicate with shinobi on the front lines or anywhere on the battlefield. Ao, along with four other sensors maintain the sensory barrier located in the centre of the room. The barrier covers the entire battlefield and works closely with the Intelligence division as they are able to detect the size of the opposing side's chakra as well as tell sheer number of persons in any given area. Additionally, Shikaku Nara was also selected to remain at headquarters as the Alliance's Chief Strategist where he devises attack, counter-attack patterns, defensive manoeuvres as well as back-up strategies.