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 the Legal Guilds from having guild wars with each other. Notably, the 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.

Fairy Tail vs. Phantom Lord
The Fairy Tail vs. Phantom Lord War was the main event of the Phantom Lord arc. It took place in the year X784 with the purpose of capturing Lucy Heartfilia. Sparked when Iron Dragon Gajeel Redfox destroyed the Fairy Tail guild and attacked the members of Shadow Gear, the war starts at the Phantom Lord Guild which was attacked by the guild members of Fairy Tail led by the master, Makarov Dreyar. However, the master was weakened by Aria's Magic, causing the guild members to retreat. Phantom Lord later attacks the Fairy Tail Guild but was subsequently defeated by the help of Natsu Dragneel and the newly-healed Makarov.

Fairy Tail Internal Conflict
The Fairy Tail Internal Conflict was the main event of the Fighting Festival arc. It took place in the year X784 with the purpose of Laxus Dreyar in an attempt to replace his grandfather as guild master. When Laxus had Evergreen petrify the Fairy Tail girls, the civil war started and the fairies were defeated by the members of the Thunder God Tribe one by one. When most of the members were defeated, Erza was freed of her curse and defeats Evergreen, freeing the other girls. Freed Justine and Bickslow were later defeated by Mirajane Strauss and Lucy Heartfilia and Laxus was defeated by the combination of the Fire Dragon, Natsu, and the Iron Dragon, Gajeel.

Light Team vs Oración Seis
The Light Team vs. Oración Seis war was the main event of the Oración Seis arc. It took place during the year X784 with the purpose of defeating and stopping the dark guild. After being initially defeated by the dark guild, the members of the Light Team manage to defeat them. Gray Fullbuster and Lyon Vastia defeat Racer while Lucy Heartfilia and Hibiki Lates defeat Angel. Hoteye was converted by Nirvana and decides to help Jura Neekis stop his guild mates while Cobra is defeated by Natsu Dragneel and Happy. Midnight, after defeating two members of the Light Team, is defeated by Erza Scarlet 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 was the main event of the Tenrou Island arc. It took 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 was incapacitated by Grimoire Heart Guild Master and former Fairy Tail Master Hades while the Seven Kin of Purgatory led the attack on the island. However, the Seven Kin of Purgatory was subsequently defeated by the Fairy Tail Mages. Grimoire Heart Deputy Bluenote Stinger also joined in the fray but was defeated by Fairy Tail Ace Gildarts Clive. Team Natsu, Wendy Marvell and the Exceeds then battled Hades with Laxus Dreyar joining in as well. Hades would've defeated the Fairy Tail Mages if the Exceeds hadn't destroyed his power source and given Natsu the chance to defeat him. However, this war led to the reawakening of Acnologia which subsequently caused the Fairy Tail Mages to be frozen in time for seven years.