Throughout the years of his career as a pirate, Whitebeard achieved many great things. From God Valley to “rivaling” Roger and even challenging the Marines and the World Government in Marineford.
However, non of those compare to his greatest achievement. You wonder what is it? I’ll tell you. Whitebeard is the guardian. Of what you may ask? Let’s go back to Roger for that.
On their last voyage, the Roger Pirates learned the truth behind the Void Century, Ancient Weapons, and the meaning of the D. They found out the truth about everything and how everything is connected.
When the crew disbanded, Roger said he wanted to talk to Whitebeard.
Before his death, Roger offered to tell Whitebeard the location of Laugh Tale and explained the significance of the initial D at their last meeting but Whitebeard refused.
In reality it wouldn’t matter if Whitebeard went to Laugh Tale or not because he wouldn’t have made a huge difference to the world as a whole. We know Roger can be persuasive, if he wanted Whitebeard to go to Laugh Tale, he would’ve convinced him.
The truth is that Roger tasked Whitebeard to protect Fishman Island. That’s Whitebeard’s greatest achievement. Making sure the Ancient Weapon is born and protected in a safe environment.
*Theory by Edward Teach