The Great War (大戦争, Dai Sensō?) refers to an intense war that took place in Europe, the stronghold of powerful ability users, namely the Transcendents and Dame Agatha Christie's Order of the Clock Tower. It took place 14 years ago and expanded influence through countries outside of Europe.
Warring parties later agreed to a ceasefire, but Mimic, one of the war's participants, received falsified orders from their own country to attack a walled city. This attack violated the ceasefire, resulting in Mimic's expulsion from their homeland, later by the Order of the Clock Tower.  At the end of the Great War, France, Germany, and England signed the peace treaty and created Standard Island as a symbol of peace.
It is still unknown what part Japan played in the war, however it is known that a few known people did participate. Mori and Yosano were part of the medical team that oversaw the treatment of the troops who were stationed on Tokoyami Island. During this war, Mori utilized Yosano's ability for his "Immortal Regiment" to prove to the upper brass of the military that Abilities should be used as weapons for war, but failed after Yosano suffered a mental break down after witnessing trauma from watching several soldiers, including Tachihara's brother, die due to the combined trauma of war and the repeated use of her healing ability. She was arrested shortly afterward for trying to blow up the army ship they were on with explosives. 
Around the same time, a lab in Germany was researching Abilities and attempted to create a Singularity without multiple ability users. The project entailed the creation of man-made ability users- artificial humans implanted with Abilities resulting in them being able to manipulate gravity. This research was shared with Japan and France who were allied with Germany in the war, and France used it to create an artificial human known as Paul Verlaine. In Japan, the military illegally abducted a child known as Chūya Nakahara and performed experiments, using his genetic material to create a clone which who was altered and made to contain an entity called 'Arahabaki'. However, Verlaine and his partner, Arthur Rimbaud, had become spies by this point and infiltrated the facility the clone- also known as Chuuya Nakahara- held in but had a disagreement over what to do with the boy, leading to a fight between the two and culminating in Chuuya's release and the subsequent destruction of the facility and surrounding area, which would later become Suribachi City.
Elsewhere, Hunting Dogs commander Ōchi Fukuchi also participated in the war at some point as a soldier of the Japanese military and also experienced horrendous trauma in combat and the loss of the war itself. Fukuzawa did not participate in the war, however, he was hired by the Japanese government to assassinate a certain number of military war hawk bureaucrats and foreign correspondents who wished to continue moving the frontlines forward, thus bringing Japan's involvement in the war to an end. 
The Hunting Dogs project is suspected to take place after the war.
- France (Mimic)
- The war is the cause of the surging numbers of orphans worldwide.
- The number of ability users legally working decreased, most of them disappeared in the public eye or started working for an underground organization.