After Gold Roger's execution and the Great Pirate Era began, swarms of new pirates cropped up everywhere. The Marines of One Piece were responsible for driving back those pirates — no small feat, given how many there are. The world was changing, and the Marines had to change with it if they were going to maintain power. Meanwhile, the Four Emperors of the New World were just beginning to establish their power and territory; the World Government created a system consisting of the Seven Warlords of the Sea in response to the growing number of pirates.
Source：https://www.cbr.com/one-piece-seven-warlords-of-the-sea/
