One Piece has hinted at the enigma surrounding the God Valley incident, yet it has never fully answered why Garp acted to protect the Celestial Dragons in that significant battle. Garp has always rejected promotions that tied him to serve Nobles, but at God Valley, his actions would seem to indicate otherwise.
A flashback could finally provide clarity, so fans could see what really pushed him to join Roger that day. The reasons behind his decision may not be related to loyalty to the Celestial Dragons at all, but something more profoundly related to his own sense of justice and morality.
Disclaimer: The article is a speculative theory and includes spoilers from the One Piece manga.
The real reason why Garp joined hands with Roger at the God Valley incident in One Piece

The God Valley incident is arguably one of the biggest mysteries of One Piece, and Eiichiro Oda has intentionally kept it a secret. While it's been established that the event involved Rocks D. Xebec vs. Roger and Garp, what has never been fully explained is why Garp did what he did, especially in regards to saving the Celestial Dragons.
This is particularly odd because Garp has always been indicated to be a man who hates the Nobles, making the choice to turn down countless offers for Admiral so he didn't have to take orders from them. And yet, at God Valley, history remembers him as a hero for protecting the Celestial Dragons. A future flashback could finally reveal the truth behind his motivation
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The accepted history states that Garp and Roger teamed up to protect the Celestial Dragons from the Rocks Pirates. What if instead of Garp's actions arising from allegiance to the Nobles, he was acting from his moral compass all along? If Rocks was targeting even children on his warpath, Garp might have acted not to save the Celestial Dragons as rulers, but instead to prevent innocent lives from being wiped out.
This differentiation would account for Sengoku's use of official terms to describe Garp's actions, but not Garp's own refusal to brag about the triumph. It also relates to Roger's subsequent discussion with Garp on Ace. When Roger requested that Garp save his unborn son, he was asking Garp to uphold his belief that children never have to pay for the misdeeds of their fathers, one that Garp perhaps already established in God Valley.

Such a reveal would shift the frame of the incident entirely. Garp would no longer look like a hesitant protector of corrupt Celestial Dragons. Garp would be a man who stood against unnecessary cruelty, even if he was protecting people he disliked.
This nuance may also explain why Garp had the title of “Hero of the Marines,” but felt no pride in the title. He knew the truth, and that truth was much more personal than how the World Government characterized Garp’s actions.
Final thoughts
The God Valley flashback could be one of the most powerful revelations in the One Piece series, as it would not only let the viewer judge Garp's character but also judge Garp's legacy.
Instead of being remembered as the unwilling savior of the Celestial Dragons, even indirectly, he could be remembered as someone who acted out of conviction to save the innocent people from Rocks' wrath. This would say a lot about his relationship to Roger and Ace, but more importantly, it simply demonstrates that Garp's way of justice has always been humanitarian-driven, not duty-based.
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