Guild War

A Guild War (ギルド間抗争 Girudo Kan Kōsō) is when two or more guilds declare war on each other and try to exterminate one another.

The Magic Council has forbidden Legal Guilds from having guild wars with each other. Though, Legal Guilds are allowed to declare war on Dark Guilds without fear of retribution from the Council, however they must still notify them of it. This is most likely done in order to lower the high number of Dark Guilds.

Prior to these events, in X679, Guild Wars were common as Legal Guilds fought over commercial rights, land and establishments.

Fairy Tail vs. Phantom Lord
The Fairy Tail vs. Phantom Lord War is the main event of the Phantom Lord arc. It takes place in the year X784 with the purpose of capturing Lucy Heartfilia. Sparked when Iron Dragon Gajeel Redfox destroys the Fairy Tail guild's building and attacks the members of Shadow Gear, the war starts at the Phantom Lord Guild where guild members of Fairy Tail led by guild master Makarov Dreyar attack Phantom Lord's guild and its mages. However, Marakov is weakened by Aria's Magic, forcing Erza Scarlet to order the guild members to retreat. Phantom Lord later attacks the Fairy Tail Guild but is subsequently defeated by the help of Natsu Dragneel and newly healed Makarov.

Fairy Tail Internal Conflict
The Battle of Fairy Tail is the main event of the Battle of Fairy Tail arc. It takes place in the year X784 with the purpose being an attempt by Laxus Dreyar to replace his grandfather as Guild Master. When Laxus had Evergreen petrify the Fairy Tail girls, the civil war starts and the fairies are defeated by the members of the Thunder God Tribe and each other one by one. When most of the members are defeated, Erza is freed of her curse and defeats Evergreen, freeing the other girls. Freed Justine and Bickslow are then defeated by Mirajane Strauss and Lucy respectively and Laxus is defeated by the combination of the Fire Dragon, Natsu, and the Iron Dragon, Gajeel.

Allied Forces vs. Oración Seis
The Allied Forces vs. Oración Seis war is the main event of the Oración Seis arc. It takes place during the year X784 with the purpose of defeating and stopping the Dark Guilds. The Allied Forces is initially defeated by Oración Seis when confronted by the entire guild, though later on the members of the Allied Forces manage to defeat them when pairing up to take down individual members. Gray Fullbuster and Lyon Vastia defeat Racer while Lucy and Hibiki Lates defeat Angel. Hoteye is converted by Nirvana and decides to help Jura Neekis stop his guild mates while Cobra is defeated by Natsu and Happy. Midnight, after defeating two members of the Allied Forces, is defeated by Erza and Brain/Zero is defeated by Natsu with the help of Jellal Fernandes.

Fairy Tail vs. Grimoire Heart
The Fairy Tail vs. Grimoire Heart War is the main event of the Tenrou Island arc. It takes place in the year X784 when Grimoire Heart interrupts the Fairy Tail Guild's S-Class Mage Promotion Trial to find the Dark Mage Zeref. During the first part of the war, Makarov Dreyar is incapacitated by Grimoire Heart Guild Master and former Fairy Tail Master Hades while the Seven Kin of Purgatory lead the attack on the island. However, the Seven Kin of Purgatory are subsequently defeated by the Fairy Tail Mages. Grimoire Heart Deputy Bluenote Stinger also joins in the fray but is defeated by Fairy Tail Ace Gildarts Clive. Team Natsu, Wendy Marvell and the Exceeds then battle Hades with Laxus Dreyar joining in as well. Hades pushes the Fairy Tail Mages to their limits and may have won if the Exceeds had not destroyed his power source and given Natsu the chance to defeat him. However, this war leads to the reawakening of Acnologia which subsequently causes the Fairy Tail Mages to be frozen in time for seven years.

Fairy Tail vs. Tartaros
The Fairy Tail vs. Tartaros War is the main event of the Tartaros arc. It takes place in the year X791 when Tartaros starts massacring members of the Magic Council. One of their members, Tempesta, happens to cross path with members of Fairy Tail when he is on a mission to kill Yajima, a former member of the Council. Their battle resulted with the defeat of the Thunder God Tribe in which ignites the fury of the other members of Fairy Tail.