The entire Navy from a villainous-world timeline transmigrates and discovers that, in another world, their counterparts are fighting in the Paramount War.
One Piece fans, be warned. No characters will be killed, but I'm a Navy fan, so the pirates won't get as much focus. The villainous Navy will bully the pirates (I won't insult or vilify the pirate characters, but that's as far as I'll go. I really love the Navy, orz).
Luffy joins the Navy, Ace is still Whitebeard's adopted son, and the entire Navy is evil. Basically, I thought things were already awesome, but wouldn't they be even more awesome if they turned evil? This isn't a mirror world. Xia Yan was added to give the changed timeline an obvious reason.
To make them easy to distinguish, the villainous-timeline characters are written in [ ], while the regular-timeline characters aren't.
The author is catching up on the manga, so there may be more details based on its timeline and setting.
Out on the sea, the [Navy] has a rather apt description:
