One of the most interesting wars in One Piece is the Incident at God Valley. It was there that Garp earned the nickname "Hero of the Marines" and also the greatest battle in the history of One Piece. At that time, Garp made an alliance with Roger to defeat the most dangerous pirate group, the Rocks Pirates, led by Rocks D. Xebec.

Xebec was the captain of the Rocks Pirates who also led 3 future Yonkous: the late Charlotte Linlin (Big Mom), Kaido, and the late Edward Newgate (Whitebeard). It is still debatable whether Roger was a Yonkou before Shanks or not. It might be possible that there were only three Yonkous and one Pirate King back then.

The Power of Three Yonkous Combined

It was stated in Oden's journal that Whitebeard and Roger's strength was almost equal in their prime. Meanwhile, Whitebeard's strength might not be much different from Big Mom's and Kaido's, even though Garp had helped them, Xebec, Shiki, and the other Rocks crew were still very strong back then.

From this information alone, we can say that normally Roger and Garp would not be able to win against them. Even though at that time Garp might be the strongest in the Marines, the strength of the two of them alone should still be far behind the rest of the Rocks Pirates.

Gol D. Roger did have a strong crew, not inferior to Rocks. At that time, Shanks was still small, but there were Rayleigh, Scopper Gaban, Crocus, and others. Their strength may still be under Roger's, but there might be a possibility that they can fight on an equal strength or even defeat the Rocks' crew.

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Mutiny in Rocks' Crew

Another possibility if Roger and Garp's alliance cannot defeat Rocks is that Xebec's crew betrayed him. It's not strange if Gorosei took advantage of this situation for the benefit of the World Government. Their manipulative nature has been seen throughout the story of One Piece.

It's also not strange if the nickname Yonkou was born after the war in God Valley. It was the World Government that gave that title to the strongest pirate besides the Pirate King. It is possible that the Gorosei had promised Whitebeard, Big Mom, and Kaido territory and a reputation so that they would be feared by other pirates.

Meanwhile, there is nothing they can achieve if they continue to follow Rocks. Makes sense right? This was because judging from the combat strength of both sides, it seemed that the Rocks Pirates still had the upper hand with such a powerful crew. Roger and Garp themselves probably knew when the three of them betrayed Rocks.

This may also be related to Marshall D. Teach who might be Rocks' biological son. Teach, who knew about the betrayal committed by his father's crew, sought revenge. He had already killed Whitebeard and maybe he would also target Kaido as well. On the other hand, Big Mom had died in the battle of Onigashima.