M. Caesar Kurang had originally been a member of the Navy's Science Unit and a colleague of Vegapunk's. He was a genius scientist second only to Vegapunk! It was not strange at all for him to appear at Marine Headquarters.
But "second only to Vegapunk" meant that M. Caesar Kurang was inferior to Vegapunk.
In Caesar's eyes, that universally acknowledged fact was an enormous humiliation!
To prove that he was superior to Vegapunk, he opposed Vegapunk at every turn. After repeated defeats and repeated attempts, his thinking became increasingly extreme and paranoid. Finally, in the year 1518 of the Sea Garden Era, Caesar caused the poison gas accident that destroyed all of Punk Hazard!
Because of that, the World Government ordered his arrest and had him escorted back to Mary Geoise to answer for his crimes.
Later, with the help of Donquixote Doflamingo, one of the Shichibukai, he escaped from the prison ship. The flamingo had his eye on the Lineage Factor technology he and Vegapunk had studied, having him produce the raw material "SAD" for the artificial Zoan Devil Fruits known as "SMILEs." He used it to trade with Kaido and seize the reins of the monsters of the New World.
Faced with Doflamingo's fists, the two sides began their cooperation. Caesar Clown also returned to Punk Hazard in the year 1519 of the Sea Garden Era and began his own evil scheme.
What happened afterward was well known to manga fans. Besides producing "SAD," Caesar swindled funding from one of the Four Emperors, Charlotte Linlin. On the surface, he had agreed to help Charlotte Linlin conduct gigantification experiments, but in reality, he used the money to make poison gas bombs. Three years later, in the year 1522 of the Sea Garden Era, the Straw Hat Pirates set out