Known Historical Record
What we do know about the origins and activities of Z.E.R.O. within the last century
Although there is much contention among details, the first suggested appearance of Z.E.R.O. was at the end of The Great War. The widely accepted theory is that the founders of the organization were mercenaries, displaced soldiers, and various civilians and vigilantes from many parts of Europe who had banded together to help with restoration efforts. The documented efforts of these individuals can lead us to infer how they evolved into the international military force we know today, before they became shrouded in secrecy.
Acting on the devastation of the war and the severely weakened national powers, many villainous organizations and individuals ran rampant across Europe, causing a major hindrance to war restoration and rubbing salt in very recent wounds. The forerunners, though not gaining official approval from national governments, aimed to quell these attacks so that a lasting peace could be had. Though the organizers are unknown, there is evidence that a chain of command was employed, and that "troops" would be shifted around the continent to provide aid where it was needed. Being an international force, they had no interest in the conquest or expansion of territory, and were concerned only with restoring the peace in any nation that required assistance.
Eventually things quieted, and incidents became less and less prevalent. Although they were never directly affiliated with the League of Nations, this international police force had gained their approval, along with the United States, Germany, and the newly formed Soviet Union. But despite being on good terms with most nations, it was plainly obvious that there were still tensions present in post-WWI Europe. Few were happy with the Treaty of Versailles, especially Germany, and there was a great feeling of discontent among many nations and factions. The forerunner organization themselves did not agree with the treaty, and were becoming very concerned with the actions of both Germany and the USSR. Furthermore, they were largely unhappy with the responses of the League of Nations, being very lenient in their enforcement of the agreements in the Treaty.
For the next ten years, very little was heard of these individuals. Their involvement in international affairs during these years was still present, but highly diminished from what it had been. Meanwhile, Germany grew more and more ambitious, venturing to annex nearby territories while the League of Nations looked on disapprovingly, but refrained from taking action. This continued for many more years until 1938 when Germany invaded Poland, and the European nations finally declared war upon Germany and their allies.
Action During WWII
It was at this point that a militaristic organization, the first in recorded history to call themselves Z.E.R.O., emerged to provide aid to the Allied Powers in the war. Seemingly from nowhere, they had amassed a small army of highly trained minds and soldiers, along with vehicles, weaponry, and various technology that was noticeably advanced for the time period. How they had acquired any of this, and how they had done so without record from any nation, is unknown. What is known is that they were integral in holding back the super-tech, meta-human squads, and supernatural phenomenon employed under the Axis' banner. This allowed the Allies to concentrate on the rest of the Axis troops, which still consisted of the majority of forces employed.
As the war progressed, the Axis powers procured more and more extra-normal troops and weaponry, either out of desperation, or simply because more time had been allowed to develop or find them. But Z.E.R.O. continued to answer back with their own resources, proving more than a match for Germany until their military defeat in 1945.
However, the Eastern Axis territories seemed to provide a greater threat to Z.E.R.O.'s forces, speculatively due to their tactics of harnessing spiritual and supernatural forces during the war. Though fighting had slowed for the Allies and negotiations were in progress, there is evidence that Z.E.R.O. did not hold back their attacks during the months following Germany's surrender. By mid-July, the same time that the United States had issued an ultimatum to the Japanese government, they had reached the shores of Nagasaki, a renowned spiritual epicenter in Japan.
What happened next is the subject of a great deal of debate. It is agreed by all accounts that the United States dropped the "Little Boy" atomic bomb on the city of Hiroshima, after Japan's refusal to surrender. The resulting blast killed almost a third of the city's population instantly, and left a large portion of the rest in critical condition. Despite this, Japan remained unwilling to surrender until their unreasonable demands were met. Three days later, on August 9th, a similar explosion took place in Nagasaki. It killed an estimated 40,000-70,000 people instantly, almost as much as the Hiroshima bomb. Japan surrendered soon after.
However, nearly all firsthand claims by those with spiritual attunement report that the second explosion was not a result of a bombing by the United States. They claim it had another origin, as it produced a sort of spiritual or psychic backlash when it went off. Aside from the fact that Z.E.R.O. was seen invading the area prior to the explosion, possible further support for this theory is reluctance of both Japan and the United States to discuss the details of these attacks immediately following Japan's surrender. If these individuals are to be believed, it may be the first and only known record of an attack by L.A.S.T. Unfortunately, this cannot be proven by anyone but Z.E.R.O. themselves, who show no interest in divulging such information.
Post WWII and the Cold War Era
Following the war, the United Nations was created, consisting of many of the nations originally involved in the recent war. Z.E.R.O. entered negotiations with the board, not wishing to be confined by the laws and restrictions set in place by the conglomerate, but still wishing to remain on good terms with them. In the end it was agreed that certain members of the UN would be privy to the inner workings of Z.E.R.O. and able to appoint directors, but for the most part the organization would control it's own business, free to act as they pleased so long as they aimed to serve and protect the planet as a whole. S.E.C.O.N.D. would act as the public face of the organization, responding to inquiries by various powers and fighting battles in the public eye as needed. F.I.R.S.T. and L.A.S.T. were stated to exist, but their actions and campaigns are still largely a mystery to the general public, along with many aspects about the organization.
There are great deals of speculation on what engagements they have involved themselves in over the years. Examples include the Korean War, the Vietnam War, the Cuban Missile Crisis, the Iranian Hostage Crisis, various disputes between Israel, Pakistan, and other Middle Eastern countries, wars and skirmishes in Africa, and the eventual collapse of the Soviet Union. It would be reasonable to assume they had a great deal of influence in all of them, and possibly several more conflicts unknown to the general public, but the only confirmed actions after all these years are S.E.C.O.N.D.'s various "peace keeping" deployments throughout the world.
Very little has changed about Z.E.R.O. in the past decades. The organization remains as tight-lipped as ever, having a discipline towards secrecy that is unmatched. Modern historians constantly argue over the nature, origin, and deeds of Z.E.R.O., and have little concrete evidence to show even after 60 years of research. Rumor and speculation have elevated them to an almost mythical status, and the populaces of many nations are still firmly opposed to their devotion to silence. But this remains an annoyance born mostly out of curiosity, rather than fear, as they have been well established in their roll as protectors.
New talent is constantly being recruited, pulling the best of the best from whatever nation the recruits hail from into their ranks. They do not discriminate based on race, nationality, gender, or even species, but staunchly demand that those who join up hold no loyalties to any other organized body of people. It is one of the main reasons that their secrets remain secret, and no leaks of information have occurred in 70 years of peacekeeping. In particular this has caused Z.E.R.O. to have a strained relationship with The Trimagus Council, as they are very opposed to having young magi taken away from their purview and control. But the relatively small number of magic users under Z.E.R.O.'s employ prompts them to look away for the most part, and so no further animosity has been achieved in recent years.
Continuing research is underway, but little is expected to be learned about the organization in a timely manner. But, considering most of our intelligence regarding them comes from their participation in one of the largest military conflicts in recent history, perhaps it is just as well to hope that no further information has need to arise.