Dragon was an ex-marine. We know, from Dragon’s Vivre Card, that The Revolutionary Army was formed between Roger’s death and Luffy’s birth. That was between 24 and 19 years ago. Knowing he is now 55 years old, that means he was not a revolutionary until he was 31–36.
His Vivre Card also hints at a possible secret about his past, namely that at a certain point of his life he realized how truly rotten the world was and became convinced that it was his duty to change the world for the better.
This leaves us with a gap between his coming of age and his being 31–36 years old. Of course, he could have been just a postman, but a marine makes far more sense for a story like his. We have seen similar stories of marines leaving the army in the past, as with Kuzan or Zephyr. I think Dragon joined the Marines and was part of them until a certain incident happened between 24 and 19 years ago. Then he left and formed the revolutionary army.
His relation to the admirals
If we analyze Marineford, we can see how Akainu never calls Luffy by his name but always refers to him as Dragon’s son. This is especially remarkable because he does it even before Segonku makes the official announcement.
When Sengoku shares it, warlords, vice admirals, and pirates all look surprised. Furthermore, Garp mentions that it is not worth hiding it anymore. We can assume the admirals knew because of their rank, but it is still an important detail that Akainu already knew.
Sakazuki and Dragon are both 55 years old. We know both Akainu and Kizaru joined the marines 32 years ago, so that gives them a period of 9–13 years to interact with Dragon. If all of them reached the rank of Vice Admiral, it is very feasible that they worked together on more than one occasion.
I do not want to go deep into Chinese mythology, because it is not my area of expertise and it is very easy to get lost in fake tales, but I firmly believe the admirals are based on the four seas and the different dragons in Feng Shui (as well as the Momotar tale).
The cardinal directions of these four seas are determined by the marker stars of the mega-constellations known as the Four Celestial Animals. Here are some details about them and their dragon equivalents:
Azure Dragon (East): blue and green, associated with birds, wood, and growth. I think it represented Aokiji (blue pheasant) and later Ryokugyu (green bull).
Vermilion Bird (South): Associated with fire, summer, and the color red, as well as good luck and fortune. It would be Akainu (Red Dog). White Tiger (West): purity, virtue, mourning, and death. I think it is Fujitora (the Wisteria Tiger), and that he went blind after a close one’s death. Black Tortoise (North): Associated with revenge and often linked to catastrophes based on water, such as floods and storms. For me, that screams “Dragon.”
The yellow dragon (center) represents royalty and the emperor, as well as authority, fortune, and wisdom. That would be Kizaru (yellow monkey). Dragon was not only a marine but a top-tier one. He was a vice admiral together with Akainu, Aokiji, and Kizaru and was meant to become an admiral as well.
The Ohara Incident
We know that something happened between 24 and 19 years ago that moved Dragon to form the revolutionary army. As stated by his Vivre Card, he realized how truly rotten the world was.
We know the Ohara Incident happened 22 years ago and that five vice admirals were involved; of them, we only saw the faces of two: Kuzan and Sakazuki. The other three remain unknown.
Knowing how it affected everyone present, I think it is an incident big enough to make Dragon resign from the Marines and start the Revolutionary Army. The world was rotten, and he had to change it.
Dragon is an ex-marine. He was a vice admiral that trained together with Sazakuki, and he participated in the Buster Call to Ohara. After the incident, he resigned from the Marines and formed the Revolutionary Army.