The Fall of the Ancient Kingdom, and the End of the Void Century
This theory is going to be a bit of a long one, but I'm going to try keep things as short as possible. The main body of the theory involves a lot of headcanon, but I'll explain all the key pieces of evidence for the theory so we're hopefully on the same page before I go in to full theorycrafting mode.
I guess to start with, it's probably a good idea to remind you guys of two key incidents within the series that will form the "core" of this theory.
Firstly, Clover's speech to the World Government during the Ohara Incident:
Secondly, the God Valley Incident:
Some key pieces of information we can ascertain from these two incidents:
> At the start of the Void Century, there was a kingdom, referred to as the Ancient Kingdom, that ruled over the world. They were a Kingdom that held advanced knowledge, and held power.
> The Void Century ends with the formation of the World Government, and the erasure of the Ancient Kingdom from the history books.
> As evidenced by the God Valley incident, the WG are more than happy to fight alongside pirates should there be a greater threat. This can also be seen in the Alabasta and Dressrosa arcs.
> Much like the Oharans, Rocks D. Xebec was interested in subjects the WG considered taboo.
> Roger's role in the God Valley incident was erased from history, as was Rocks' existence.
> The Government only directly fought Xebec during the God Valley Incident. The key rivalry was Roger vs. Xebec, rather than Xebec vs. the World Government.
Another key concept that I want to highlight before we go into the theory is the idea of Peacemains vs. Morganeers. It's not explained in the main series, but Oda brings it up in the Romance Dawn oneshot:
The most important takeaway from this is that pirates can be divided into two groups based on ideology. We've seen it in the One Piece story, most notably with Roger vs. Xebec, and also with Luffy vs. Blackbeard. Whilst Blackbeard hasn't had much screentime, his actions and his portrayal as Luffy's direct opposite suggest that whilst Luffy is the "model" Peacemain, Blackbeard is the "model" Morganeer.
With these pieces of information, it's time for me to outline the main hypothesis of this theory. That is, that the major conflict of the Void Century was a battle between pirates. Namely, it was a conflict between Peacemains and Morganeers. This theory will go into detail and explain a possible chain of events that eventually led to the end of this conflict, the fall of the Ancient Kingdom and the rise of the World Government.
The Ancient Kingdom - A Kingdom of Pirates
Before I go into detail about the events of the Void Century, I think it's important to determine who lived in the Ancient Kingdom, and why it eventually fell. I believe the Ancient Kingdom was a Kingdom filled with Pirates, most of whom were Peacemains. This assertion is backed up by the existence of Joy Boy, who in Oden's flashback we find out lived in the Ancient Kingdom.
From both the Laugh Tale segment of that flashback and Robin's side mission in Fishman Island, we know Joy Boy was responsible for the content on the Poneglyphs.
Now, why does this lead me to believe the Ancient Kingdom was filled pirates? Well, it boils down to the dispersal of the Poneglyphs across the world. In a time with no Marines, the only group of people that we know of that could have travelled the seas are Pirates. Joy Boy himself may not have been involved in this journey (he could have been dead for all we know), but he could have had people from his Kingdom to go across the world and spread his message in such a way that it could never be lost. He would also have had a say on where to leave the Poneglyphs - likely in the hands of people he trusts.
Why do I think they were Peacemains? Well, that's quite easy to explain. Joy Boy's wish to bring the Fishmen to the surface isn't the ambition of a Morganeer, who would likely have tried to conquer the island. I think Joy Boy's ideal world, and the message he left behind on the Poneglyphs was one of a free world without discrimination.
A Quick Note on the Old World
Another important point to raise is that of the state of the world before the formation of the World Government. We know that 20 Kingdoms went on to form the World Government, and, save for the Nefertari Family, their royal families eventually went to Mariejois and became the Tenryuubito. So what was the world like without a World Government? Well, I believe that most countries were independently run by their kings, and weren't overly connected to each other. Basically, like the current Wano Country, but with open borders. There may have been small unions of countries that were geographically close to one another, but that would be as close as it came. The Ancient Kingdom was likely the only country with the means to travel the world, and their era was likely similar to that of Columbus and the other privateers who travelled the world, minus the horrendous colonisation. Well, at least it was that way for a while.
The Rise of the Morganeers, and the Beginning of the Ancient Kingdom's Downfall
So, what we've established so far is that the Ancient Kingdom was an island full of peace-loving pirates. Those connected to Joy Boy, who was likely a senior figure in that Kingdom, likely shared his ideal of a free world. We also know that the Ancient Kingdom were the dominant power in the world at this time. They were able to roam the world, and adventure freely. However, absolute power corrupts absolutely. Whether it was the voice of one man, or a string of incidents, a faction of pirates from the Ancient Kingdom one day realised "hey, we can kinda do whatever we want and get away with it." This ideal led to pillaging, conquering et cetera, and eventually the birth of the Morganeer ideology. This would have formed a major rift in the Ancient Kingdom, with Joy Boy and the Peacemains being strongly opposed to this way of living, whereas the Morganeers think it's fine since they're strong and that their strength is justice. This would eventually lead to the Morganeers wanting to conquer and form an Empire run by pirates who share their beliefs, and this greed and lust for power would be the beginning of the end for the Ancient Kingdom.
The Ancient Pirate War
Without a World Government, the Morganeers were pretty free to go from island to island, conquering and generally doing as they please. This was dangerous for most countries, as there was likely a major difference in strength between the Morganeers and each country's military/defence system. So, who would help the countries fight against the Morganeers? Well naturally, the Peacemains. They would go from country to country, helping each nation fend off the Morganeers, eventually driving them out. This would continue until the Morganeers either gave up, or were wiped out to a point where they could no longer fight. The Peacemains were likely not bothered with accolades or praise (see: Luffy), and left the world to its own devices. However, this would end up being the final nail in the Ancient Kingdom's coffin.
The Formation of the World Government, and the Criminalization of Piracy
The war is over, and the 20 Kingdoms were saved. But, the conclusion of this war brought about one painful realisation - they were weak. If only there were some way that they could form a global-scale alliance to protect each other should a war like this ever happen again....
And that's how the World Government was born. The 20 Kingdoms formed this alliance, which would go on to be the WG we see in the present One Piece universe, and its heads (or 19/20 of them, at least) moved to Mariejois so they would be safe from any pirate attacks in the future.
With the World Government now formed, there was only one more topic left on the agenda - what to do about the pirates? Sure, it was pirates that ended up saving them, but how would anyone know if a pirate landing on their shores was good or bad? The conclusion they reached was that for everyone's safety, pirates were no longer allowed. The WG would pool together its resources and its strongest fighters, and launch a mission to eradicate the people of the Ancient Kingdom. Not only would they target pirates with their own armies, but they would offer cash prizes for anyone that managed to catch a pirate and hand them over. This would eventually evolve into the bounty system we see today.
Oh, and if you think they wouldn't go as far as genocide, I will refer you to the story of Portgas D. Rouge and the lengths the WG went to extinguish Roger's bloodline. That ideology was likely born during this purging of the Ancient Kingdom.
The End of the Ancient Kingdom, and the "Will of D."
With the war on pirates underway, the Ancient Kingdom realised their time might be coming to an end. Joy Boy and team were unable to complete their dream of creating a free world thanks to the actions of the Morganeers. However, they didn't lose hope that one day, someone would show up and complete their dream for them. Joy Boy would head to the Wano Country, and ask the Kozuki clan to write a series of messages for people in the future that would never be lost. This resulted in the birth of the Poneglyphs. The only people who could read the message on them were those of the Ancient Kingdom, whose language is likely the one we see on the Poneglyphs, and the Kozuki Clan, who would have been taught how to read and write in the language in order to construct the Poneglyph. Among the Poneglyphs created, information about the events of the war, the formation of the World Government and the location of Ancient Weapons (which haven't featured much in this theory but were likely a key part of the Ancient Kingdom's power) were inscribed, as well as instructions for how to reach their kingdom, which for the sake of protection was hidden from the rest of the world. Joy Boy would also apologise to the king of Fishman Island for being unable to fulfill his promise of bringing the fishman tribe to the surface. These Poneglyphs were then carried across the world, and left in the hands of people that Joy Boy trusted.
With their message now safe, there was one more plan enacted by Joy Boy and co. That plan was to make sure any survivors of the World Government's assault on the Ancient Kingdom were able to identify each other, and make sure Joy Boy's will was passed on. This led to descendants of the Ancient Kingdom incorporating the initial "D" into their name. I couldn't tell you exactly what the "D" stands for, but here's two possible ideas:
1. The name of the Ancient Kingdom
2. The will to bring about the "Dawn" of the world. It's a concept that's come up more in recent chapters, most notably with Roger and Pedro.
However, the people who took on the initial D. weren't all people who sided with Joyboy. Survivors of the Peacemain vs. Morganeer war who shared the Morganeers' ideology also took the name as a way of identifying each other. Whilst the World Government and the Celestial Dragons remember the meaning of this name (leading to D.'s being referred to as the "enemy of the Gods"), the meaning is lost to time. This is evidenced by the present day D.'s being mostly unaware, and just carry the initial as part of their names. An exception to this is the Trafalgar D. clan, who hide the initial.
A Quick Note on the Nefertari Family
One group I've left out of this discussion are the Nefertari family. Whilst they were involved in the formation of the World Government, they didn't move up to Mariejois. The reason for this is fairly simple - the Nefertari family were close with Joy Boy. They understood the need for kingdoms to be protected, but didn't agree with the extents the WG were going to for their goal. They were also against the idea of moving up to Mariejois and living as "Gods". Of the twenty kingdoms that formed the WG, they were the only ones to remain supportive of Joy Boy and the Ancient Kingdom. To protect his message, they remained in Alabasta. There's also an element of self-protection here - as the WG likely would have killed anyone who had connections to the Ancient Kingdom, as well as Ancient Kingdom residents themselves.
I know there's a lot to take in here, so here's a summary of the key points of this theory.
> The Void Century consisted of two wars. The first was between two rival factions in the Ancient Kingdom, the Peacemains and the Morganeers. The second was between the newly-formed World Government and the survivors of the Ancient Kingdom.
> The Peacemains won the first war, and the conclusion of the first war would lead to the first incarnation of the World Government. The World Government would eventually go on to win the second and become the establishment we see today.
Should this theory be correct, it has interesting implications for the current story. It sets up the Luffy vs. Blackbeard conflict as a repeat of the Ancient Pirate War and of the God Valley Incident, and hints at the idea that the Marines may eventually team up with him. However, unlike the events of 800 years ago, there will be forces in place (the Revolutionary Army, for example) that will ensure that the Marines who try to censor Luffy's role in the war are stopped, thus freeing the world from the Government's grasp and lettting the "true history" become common knowledge.
I hope you've enjoyed reading this theory as much as I've enjoyed writing it!