The most popular reason in the OP community is that they have done it out of fear for the ancient weapons that the ancient kingdoms had. However, knowing that somebody has a strong weapon is not the same as they know how to use it or if they are willing to use it. Plus from what we know only specific people can use these weapons.

So why did WG decided to act in that moment of history?

One of the options is that they wanted to destroy these weapons so they cant be used against them.

Second option is that they wanted these weapons for them in order to control the power (similar to Vegapunk and the lineage factor)

However, knowing that these ancient weapons have survived during time and history, I am more convinced to believe that WG wanted that power for themselves. But if the WG had such information about the ancient weapons they should also had the knowledge that such weapons can be used only by specific individuals. So how were they planning to use them if they manage to get them?

The Voice of All Things​

Well here it comes the role of the discovery of “The voice of all things”. Somebody in the ancient kingdom has made a discovery on first how to hear and then how to use “The voice of all things”. So if knowing how to willingly use the “The voice of all things” can you control these ancient weapons? May be yes, but that is not the only reason for the start of the war.

Imagine what happens if all intelligent creatures unite under a common cause living together. Equality is born and in that the rich and powerful will not be rich and powerful for long after that and they will lose their privileges. So the 20 kingdoms leaders and in my opinion later called the “Celestial Dragons” wanted to destroy that unification and the occupy the power of “The voice of all things”.

So before the knowledge can spread the WG had to extract the information for themselves, destroy all who knew about it and races had to be divided and not “understand” each other.

I place “understand” in quotation marks because obviously in one piece world everybody speak the same language in order to understand each other. However, by “understand” I mean the ability to “get along”, so in that relation the division of races had to happen. Which happened with time after the WG took control and the hate and discrimination was born duo to this isolation of races.

Thinking about this powerful knowledge that was discovered, the isolation, the separation and the destruction that followed reminds me of the myth for the Tower of Babel.

In the myth, with the Tower of Babel humans were united no matter of their races and “the whole world had one language” as the story says word for word. By building the Tower of Babel humans were trying to reach the heavens so everybody can see it and live in it. They wanted to reach their dreams, share it with everybody and be closer or equal to God. However, that was overambitious and it angered the gods so they have punished them with the destruction of the tower and the separation of the languages, so they will not understand each other anymore.

In One Piece the Tower of Babel was “The voice of all things”. The people who made the discovery had the idea of peace and uniting all intelligent creatures together so they can live in peace and equality but they were overambitious that everybody will share their understanding, because this very same thing can be seen as an unification factor by one, but it can be seen as a weapon by others for controlling the ancient weapons.

So to put it straight the war between the 20 kingdoms and the ancient kingdom was to conquer the knowledge and respectively protect the knowledge of “The voice of all things”. 

When the outcome of the war was getting close, the ancient kingdom destroyed all records on how to use “The voice of all things”. What was left was only the knowledge of the creator and his/her subordinates.

Knowing that they must escape or die protecting this knowledge a group was able to escape to a different island and this island is currently known as the Wano Kingdom!!! Part of this group could have been royals from the ancient kingdom with the name of D.

The Poneglyphs

The people who discovered “The voice of all things”, started to create the ponegliff in order to record everything that has happened and they have used the power of “The voice of all things” to tell the history through these stones. At that point they have used the writing characters known in the ancient kingdom to engrave the words but they have also made a backup plan knowing that the language might disappear through time.

In my opinion the founders of Kozuki clan have used the power of “The voice of all things” to record verbally their words which they were writing on the stones. So the ponegliff are not only written version of the history but they are an “audio” version as well for the one who is able to hear “The voice of all things”.

So the currently known Kozuki clan have discovered the voice of all things and they were able to put their thoughts on the ponegliff like a recording (audio tape/gramophone record/Compact Disc). We know that the Japanese translation of the word has a meaning of Pōnegurifu – Text of History.

But lets break down as well the word “PoneGliff” from the official translation.

On purpose I have separated this newly invented word by Oda into two – Pone and Gliff.

What the word “Pone” is similar to our language is the word “Phone” which is used in many devices all connected with audio representation – megaphone, telephone, and gramophone.

What the word “Gliff” is similar to our language is the word “Glyph” which has a meaning of a “hieroglyphic character or symbol”.

So when you put together these two separate words it sounds to me as a new word describing an Audio Hieroglyph which matches exactly with my statement that the ponegliffs are also an audio version of the history.

Even further it was confirmed that Gol D Roger was able to hear the ponegliffs and hearing the voice of all things from them. And I remind you it is specifically stated by BM that he used a power that involved HEARING the voice of all things.

At this point you might wonder why I am emphasizing on the “hearing”. I am separating the ability to use and hear the voice of all things. The ability to use is connected to the ability to put your own words as an audio record and the ability to hear is connected with hearing that specific record. Also if you go back in the text you will see that every time I have written down which power has been used I have underlined the word to make a small game with You to see if you were able to understand in which direction I was pointing :DSo did you get it before that paragraph? Anyway this is not the end, we have to continue.

To put more weight in my statement that these are not simple writings but audio recordings as well I would like to point out that in the explanations given in Zou about the ponegliffs, the description words for the ponegliffs are written in quotation marks.

Even further it is described that Kozuki clan are masters of stone carving but it is interesting what Franky is saying about the their ability. “Even now, their technological prowess is quite somethin’, if I do say so m’self”. It is interesting that Franky needs to make that comment. It almost seems like that he is seeing the ponegliffs as a piece of technology which matches with the description that I am trying to convince you, that these are also audio recordings of the history.

So to put it in short the ponegliffs are not only written version of the history but an audio version as well created by using the voice of all things. Then the knowledge of how to use it disappeared (or did it!?) and people with natural born talent, descendants of the original discoverers, are able to hear it. 

After the ponegliffs were created and the war was over and the years have passed it was given as a legacy for the next generations to spread the ponegliffs across the world and each descendant took a ponegliff with himself/herself to a different locations in the world.

One can argue that in the ancient kingdom some of the races were united or living together and in the group that escaped from the war, there were at least several races in that group. With that in mind the next generation took the ponegliffs and brought them to the places where their races have made home after the separation.

That’s why we see ponegliffs in different locations. Having in mind that probably a member of the D lineage was among these people, that’s how the D started spreading again.

The Will of D​.

After the war was over the royals from the 20 kingdoms which were the winners of the war adopted the name of the Celestial Dragons. But how was this name created? Lets break down again the meaning of that name.

Celestial – one of the meanings for celestial and in my opinion mostly related to this speculation is the comparison with “heavenly”. Does this sound similar to the Babylon tower trying to reaching the heaven and the world was punished for that.

Also the other meaning of “Celestial” is superiority over the common or the known.

So why did the new royals called themselves “Celestial Dragons”? In my opinion is to show their superiority over others. But not only over the common folks but also above the ancient kingdom as well. How is that connected? Well I think that the royals on the ancient kingdom were called Dragons and that’s why the new royals called themselves The Heavenly Dragons/ The superior dragons (Celestial Dragons).

Thinking about it the name Dragons fits as well with the letter “D” that WG fears so much. Because the people with D are the True Dragons!

Thinking more about it we know that Monkey D Dragon also called Dragon the Revolutionary is the most dangerous and most wanted man in the world.

Following this line of thoughts Dragon the Revolutionary sounds to me like the Revolution of the Dragon (True Dragons). So could actually Monkey D Dragon be a direct descendant of the Royal family in the Ancient kingdom? Could he be direct descendant of the King of the ancient kingdom? Wont that explain how the whole lineage of Garp, Dragon and Luffy have such a big potential and talent for enhancing their strength? Is that the reason why people are drawn to them so easy and find it easy to follow them? Its good food for thought :p

Continuing in the same direction the Will of D fits as well with the statements until now. To carry the will of the True Dragons to unify the world in peace which were the true intentions when the discovery of “The voice of all things” was made.

Curious thing is that now Momonosuke ate a dragon fruit and if we follow the speculations until now the Kozuki clan are descendants of the ancient kingdom. Which means that they were supporting the royals (True Dragons) in the ancient kingdom and the funny part is that now Kozuki clan and the lord of Kozuki clan (Momonosuke) is in an alliance with two of the known D. (Luffy and Law)

Gol D. Roger​

Talking about the Will of D and unifying the world what happened when Gol D Roger found about that from the ponegliffs. What happened when he finally received the legacy of his ancestors? He wanted to fulfill that legacy but he was sick and had only 2 years to live. Plus in the eyes of the common he was the worst of all, he was a pirate! The was The Pirate! Convincing the common that he wants to bring them true freedom and break the chains of inequality would definitely take much longer than the time he had left to live, due to his reputation.

So what was left for him was to entrust that to the next generation as his ancestors (The True Dragons) did before him. So he surrendered and inspired the people on his execution with the hope that someone after him will go on the right path. Someone who is building relations with different people, races and status, someone who always shows his pure intentions to right the wrongs, someone who people can follow with ease, someone like Luffy!

Furthermore he named the treasure One Piece and there is a reason for that. What he meant is not “One Piece” as everyone assumed, but “One Peace” which was the legacy of his ancestors, bring a unification (One) and peace (Piece) for everyone.

However, before surrendering he did one last thing!

On Raftel on the final ponegliff he left a message, a voice message for the one who reach it. That’s right, Roger found out how to use“The voice of all things” and recorded his own thoughts in the final ponegliff. That’s the reason why Whitebeard said that Gol D Roger is waiting for someone, also Gol D Roger said that he will not die!

He will be forever recorded in history in these indestructible ponegliffs, the same way his ancestors are.

However, this is only a pure speculation until we remember that Gol D Roger knew how to write in the ancient language! In Skypiea Robin found a writing from Roger.

To sum up Roger has left a message for the next generation and he is waiting the person who will carry the will of the True Dragons and bring One Piece (One Peace) to everyone.

If you have reached that point I would like to thank you for reading this long speculation.
It took me quite some time to come up with it and get all the connections so I hope you liked it.

*Theory by Capt_NakedPanda