For years, I always wondered why Shanks a man who would become a yonko would lose an arm to a sea king, a sea king which isn’t even that impressive compared to other sea kings?
In Oden‘s flashback, it was said that Roger left Laugh Tale and said his son would find the One Piece. Rayleigh said “but you don’t have a son Roger “, Roger “but I will”. It’s shown through context that he specifically wanted a son like he was a few months from death and all of a sudden he wanted a son?
Anyway, he went off and met Rouge who also happened to be a D clan and that’s important, but that’s a different theory. He went off to get executed and I believe the idea was for him to meet Garp as Roger didn’t plan on doing the whole “you want my treasure“ bit. Roger wanted to meet Garp because he couldn’t protect Ace.
Now, before he left Oden stated Shanks sought out Roger one day and Oden noted that Shanks suddenly began to cry, but didn’t want to stick his nose in other people’s business.
Now what was said between these two?
When Rouge died, she said her son is named Ace, and years later we find out Roger’s sword is named Ace, and that is no coincidence.
At the start of the series, Shanks lived on Dawn Island for about a year, which is very odd for a pirate, especially if that island was the home of his former captain’s greatest rival (besides Rocks)
I believe this is what happened. Roger believed his son would be Joyboy, and to create Joyboy he had to have a son with another will of D clan member (that’s another theory again )
Before he left, he told Shanks his plan and said it was up to Shanks to seek out his son and teach him the ways of a pirate. He said to find Ace he has to find Dawn island, and seek out a child around a certain age who is named Ace after his sword. Roger gave Shanks the straw hat like Joyboy’s straw hat, and asked him to give it to Ace. But the important thing is that all former Roger crew mates would see the hat, and know they should help Ace on his way one way or the other.
Then he also has to find the Gum Gum fruit as that was the fruit owned by Joyboy, so Shanks went to Dawn island and went looking for Ace. However, Garp didn’t want Ace to become a pirate, he put him in the care of bandits, so they can teach him to be a honest hardworking man who obeys the law ……genius!
Shanks traveled the island, but never thought of looking for Ace among the bandits. Eventually, he discovered where the fruit was and went to retrieve it. During his stay, he befriended luffy.
Luffy reminded Shanks of Roger, but Shanks knew Luffy wasn’t Roger’s son. Then one day after raiding government ship, he put the fruit down and Luffy ate it which made Shanks freak out.
After seeing Luffy defend his honor, Shanks made up his mind to make Luffy the successor. Shanks believed Luffy could do it, which is why he said to Whitebeard he bet his hat and arm on the future.
So why did he lose an arm?
I believe this was a extreme punishment for Shanks. He disobeyed his captains orders and to pay for what he did he sacrificed his arm. Ace was suppose to be Joyboy, but Luffy took the spotlight from him.
Theory by MrBushido56 (https://www.reddit.com/user/MrBushido56)
