Guild War

A Guild War is when two or more guilds declare war and try to exterminate each other. The Magic Council has forbidden the legal guilds from having guilds wars. Notably, the legal guilds are allowed to declare war on Dark Guilds without fear of retribution from the Council. This is most likely done in order to lower the high number of Dark Guilds existing.

Fairy Tail/Phantom Lord War
After Lucy Heartfilia joined Fairy Tail, Phantom Lord master Jose was hired by Mr. Heartfilia to bring her back home. However, Jose used this as an excuse to finally get his "revenge" on Fairy Tail and thus he began to instigate a Guild War. After pushing Fairy Tail master Makarov to the limit, Makarov declared war on Phantom Lord.
 * Participants: Fairy Tail Guild and Phantom Lord Guild
 * Noteable Participants: Makarov, Natsu Dragneel, Gray Fullbuster, Erza Scarlet, Mystogan, Jose Porla, Gajeel Redfox, Juvia Lockser, Aria, Totomaru, Sol.
 * Outcome: Phantom Lord loses and is disbanded by the council. Jose loses his status as one of the Ten Wizard Saints.

During the battle, Makarov's magic was drained by Aria after Jose used a cheap trick and leadership of Fairy Tail fell on Mirajane, Cana Alberona, and Erza. As Phantom's robotic guild attacked, Natsu, Gray, and Elfman went inside and defeated the Element 4 with Erza's help. However, Jose's arrival meant defeat for them. In a fierce battle, the two Dragon Slayers Natsu and Gajeel fought to end the war with Natsu as the victor. Despite losing his best men, Jose still wouldn't surrender. When all hope seemed lost, Mystogan retrieved all of Makarov's lost magic and destroyed all of Phantom Lord's subdivisions thus allowing Makarov to return and use Fairy Law to defeat Jose and win the war.

After the war, the council tried both Makarov and Jose over the war and damage done and they, begrudgingly, only ruled in Fairy Tail's favor on the grounds of self defense. For instigating the war, Jose was stripped of his Wizard Saint title and Phantom Lord was dissolved.

Fairy Tail Civil War

 * Participants: Fairy Tail wizards against Fairy Tail team Raijinshuu
 * Notable Participants: Natsu, Gajeel, Mystogan, Laxus Dreyar, Team Raijinshuu
 * Outcome: Natsu defeats Laxus and Laxus is excommunicated for instigating a fight and threatening innocent people.

Laxus's anger with the carefree Fairy Tail wizards reached its peak and he declared a civil war to make himself the Guild Master. After Evergreen turned a number of women to stone to act as hostages, Laxus announced the rules of the battle and thus systematically forcing the mages of Fairy Tail to fight each other while Makarov, and Natsu and Gajeel surprisingly, couldn't do anything to stop it.

As Natsu and Gajeel helped free Erza (and promptly receiving a beating for touching her) she finds and defeats Evergreen which frees the other hostages. Not to be outdone, Laxus activates the Hall of Thunder spell to keep the balance of power in his favor. Lucy then finds and defeats Bickslow with Loke's help. After this, Juvia Lockser, and Cana are defeated by Raijinshuu captain Freed Justine and Mirajane reawakens her lost powers, beats him, and makes him express his disapproval of Laxus's plans. With this, he surrenders and Laxus is left with no one to defend him.

Having enough, Mystogan arrives to talk sense into Laxus, but as he sees there is no hope for him, he engages him in combat and appears to match him. However, once Natsu and Erza arrive and his mask is blown off, he leaves the fight to Natsu as Erza goes to destroy the Hall of Thunder with the other Fairy Tail members help. Natsu is soon joined in his battle with Laxus by Gajeel and as they battle him, they discover Laxus's horrible secret: Artificial Dragon Slayer powers. As Laxus uses Fairy Law to a failed outcome, Natsu enraged pummels Laxus into submission and becomes the strongest mage in Fairy Tail.

As everyone healed and prepared for the Fantasia Parade, Luxus was judged by Makarov for the civil war. For endangering not only the lives of his comrades, but the people of Fiore, Makarov excommunicates Laxus from the guild. Before he left, the whole guild showed the now humbled Laxus that no matter how much of a jerk he is/was, they will always look out for him.

Light Guild/Balam Alliance War
While not an official war after the council's dissolve and the rise of the Dark Guilds, four legal guilds formed an alliance to stop one of the strongest Dark Guilds, the Oración Seis. Delegates from Fairy Tail, Blue Pegasus, Lamia Scale, and Cait Shelter have banded together into an alliance known as the Light Team.
 * Participants: Fairy Tail, Blue Pegasus Guild, Lamia Scale Guild, and Cait Shelter Guild vs. Oración Seis
 * Notable Participants: Light Team, Oración Seis, Jellal Fernandes
 * Outcome: Light Team victorious, Oración Seis Guild dissolved, Jellal imprisoned

Fairy Tail/Grimoire Heart War

 * Participants: Fairy Tail's S-Class Trial participants against the Grimoire Heart guild members.
 * Notable Participants: Fairy Tail's S-Class Hopefuls, Seven Kin of Purgatory, Zeref, Bluenote Stinger.
 * Outcome: The war is currently held.