The Diclonius War was an event in the Elfen Lied universe that almost saw the near-extinction of the non-horned humans in favor of the emergent Diclonii, purportedly the next step in bipedal hominid evolution. It is only described very briefly in the final chapter of the manga series, but certain events can be extrapolated from these statements and further events speculated on using them as well.
"Four months after the lighthouse incident, horned children were born around the world due to a strange disease. The WHO (World Health Organization) forbade the birth of any children before the vaccine was fully developed. Nevertheless, mankind would go extinct if they didn't reproduce. As the horned children grew, they began to use their powers to kill humans. During that time, mankind was really heading towards extinction. Still, that assistant (Arakawa) developed the vaccine in the blink of an eye. She was revered as a savior around the world. But somehow, she didn't seem very happy about it. If all goes well, the WHO will probably remove the birth restriction next year. But, it's said that Earth's population will decrease significantly. But mankind will overcome this crisis somehow." (Lifted directly from scanlations of Manga Chapter 107, Conclusion.)
Kouta's statement places the birth of Diclonius children on a massive scale at four months after the deaths of Lucy and Chief Kakuzawa. This fits with the six-month gap in manga narrative that occurred just after the death of Mariko Kurama and the enactment of Chief Kakuzawa's master plan, which seeded the Diclonius virus around the world. It is difficult to ascertain what the rest of the world made of these births, since Japan was very reluctant to admit its early role in sponsoring the Kakuzawas, and would not wish to admit knowledge of these horned children. Since typical Silpelit children do not gain their vectors before the age of three, it can be speculated that Kakuzawa had his weaponized virus altered to either age these new ones faster or to gain their abilities earlier. Given the extensive cloning that was undertaken upon Mariko, the use of her DNA in making these new Silpelits even more powerful is not out of the question. Another possible problem for the besieged human race could have been posed by pockets of previously born Diclonii children who somehow escaped being euthanized, providing shelter and guidance for the newborns. The manga series states that Kouta went back to the spot where he first met Kaede for ten successive years after her death. By the time of his final visit, he is in the company of his young daughter Nyuu, who appears to be at least six and perhaps even eight years of age. If the Silpelits born from Kakuzawa's mass infection took three years to begin their attacks, and this was followed by a birth ban of unknown duration, young Nyuu being even at the age of reason during her father's tenth visit to his meeting spot is next to impossible. For the war to operate within the events shown, it seems most likely that the usual 'rules' involving Diclonii and Silpelits had been pushed aside by the schemer behind these events.
The World Health Organization, an agency of the United Nations, does not have the power to institute a birth ban. At most, it could only recommend such a policy be instituted and make it clear to the member states that such an extreme measure was in fact needed. Some nations might at first, owing to political goals, religious beliefs or local traditions, refuse to enact or obey this ban. Ceding such sovereign authority to an outside agency is anathema to many others, including the United States and China, both of which have chafed at international treaties perceived to have mechanisms that override their nation's power over its own citizenry. International cooperation may have also been stymied by mutual fears that someone else would try and further weaponize the Diclonii for their own use. Since normal bullets are less of a threat to Diclonii, it can be assumed that the armies of the world followed Bando's lead and used heavier-caliber bullets and projectiles that were harder for some vectors to turn away. It can finally be assumed that mistrust and recalcitrance among nations began to fall away when the staggering numbers of dead Kouta described began to emerge. Whether Japan shared the secrets of its Vector Attack Craft is wholly unknown. Without a ready supply of captive Diclonii to harvest, building more of these might prove difficult.
Damage and Casualties
Kouta speaks of a war that brings humankind teetering on the brink of extinction, but at no time does his brief narrative make it clear what combination of deaths caused by Diclonii and the birth ban pushes this along. It is never revealed whether any of the main or central cast members besides Kouta survived the war. While Nyuu Junior looks very much like a young Yuka, the blood relation between the two allows for the possibility of a resemblance between them and their offspring by other people. Also unknown is if Nana was kept safe in a climate that would be openly hostile even to a peaceful Diclonius. It is unknown if Japan was able to keep its role in sponsoring Kakuzawa a secret. The exposure of this role would have made Japan an international pariah, perhaps eclipsing anti-Japanese feelings many Asian countries hold as a result of Japan's role in the Second World War. In addition to outright deaths, many non-horned humans may have been maimed or injured in some way as to frustrate reproduction. Since castration (as opposed to reversible tubal ligation) was once the demanded method of ensuring a human male infected with the Diclonius virus did not sire Diclonii children, many men may have been permanently unable to have children, having the procedure done in the panic that followed the continual attacks. Attacks by Diclonii may have also led to difficulties transporting food, medicine, and aid to the injured, exacerbating injuries and illnesses that brought about sterility or difficulty in siring and conceiving. Human casualties would have had to have been at unthinkable numbers for extinction to even be a real possibility. The areas with the highest populations would have been the most affected by such devastation, especially since the instance of parents protecting rather than turning over their horned children would be magnified and made more difficult to enforce against by a large population. It is completely unknown how factors like religion, culture, politics and economics helped or hindered the war effort, or how the world's balance of power might have changed in the face of such a radical turnaround in population growth. Yet at the end of it all, Kouta still lives in Kamakura with his family, the landscape appears the same, and he feels safe in walking around with his young daughter. It could be speculated that the Diclonii left the Kamakura area alone out of some genetically-transmitted information that it was the birthplace of Lucy, their 'Queen', but again too much is unknown to even vaguely confirm this. One final speculation is that the war was of both shorter duration and was less damaging than Kouta relates, with his accounts bound up in media stories of a period of war and panic, which might relate the very worst of the available information without the filter of journalistic standards.
In conclusion, whether Kouta related an accurate or a sensational account (which would be out of character), it seems there was a war of great devastation brought about by the mistaken racial theories held by the Kakuzawas and brought to fruition by the plans they drew against the world. The series simply does not provide enough information to draw more fact-based conclusions.