The conflict in God Valley is one of the most anticipated fights in One Piece history. In a conflict unlike anything fans had seen before, Monkey D. Garp earned the title of "Hero of the Marines" as some of the series' biggest hitters squared off. It's also the location of the alleged alliance between Gol D. Roger and Garp to take down Rocks D. Xebec, the most formidable and deadliest pirate to sail the high seas in recent memory.
Xebec was the Captain of the Rocks Pirates, which included three of the four previous generation Yonk: Edward Newgate, commonly known as Whitebeard, Kaido, and Charlotte Linlin. It's debatable if Roger was the fourth Emperor of the Sea before Red-Haired Shanks took over, or if there were three Emperors and one Pirate King. Nonetheless, Xebec must have been a great warrior to command the Strongest Man, Lady, and Monster. It seems impossible that Roger and Garp could overcome him and the rest of the Rocks Crew on their own, but perhaps they did with some assistance.
In One Piece: Measuring the Emperors' Might
It is mentioned and shown during Kozuki Oden's journal-flashbacks that Whitebeard and Roger were almost equal in strength during their primes, and were friendly rivals. On this information alone, it seems peculiar that Roger could compete with all of Whitebeard, Big Mom and Kaido at God Valley. Even with Garp by his side, there was still Xebec, Golden Lion Shiki and the rest of the Rocks Crew to fight. Surely, Whitebeard alone would have been enough to keep Roger busy for days.
Considering Garp’s power, there is little doubt that he was the strongest marine in his prime. His strength would have waned somewhat in age, but not entirely. This is likely why he was given so much freedom. However, even when combining his perceived strength with Roger’s, the pair would surely have their hands full with the future Yonkō -- let alone Xebec, who is implied to be stronger than the lot of them.
A captain does not create such a fearsome crew if he cannot keep them in line. This leads many One Piece fans to believe the events at God Valley were not so cut and dry, but in fact full of deceit and betrayal. Xebec was too powerful and dangerous to remain the ruler of the sea, and everyone knew it -- including his crew.
Xebec May Have Been Betrayed by Whitebeard, Kaido, and Big Mama
It's not implausible that the World Government's Five Elders took advantage of the situation, given how their manipulative puppeteering is already visible in One Piece. It's also no coincidence that the Yonk titles were founded after the God Valley Incident, and were given to those who'd been under the command of a pirate who challenged the World Government's long-held authority. It would be more logical to toss everybody who served Xebec into Impel Down and permanently destroy their reputation. Instead, power, status, and territory were bestowed upon Whitebeard, Kaido, and Big Mama.
All of these evidence suggests that either the future Yonk turned on Xebec and helped destroy the Rocks Pirates, or they did not get involved at all. Narratively, it makes no sense for them to have fought Garp and Roger, lost, and then been given Emperor positions or allowed to capture territory after their crew was annihilated. If they were promised their positions as Yonk in exchange for Xebec's treason in God Valley, it makes logical that they have stayed prominent in One Piece's world.
The possible reasons for betrayal would vary -- perhaps they saw Xebec as too dangerous, wanted to surpass him or simply wished to build their own world. In the end, this direction of thought makes far more sense than believing that Garp and Roger were able to overcome them all alone. If they were, they were the strongest beings to have ever walked the One Piece world, which could also be true. The idea of betrayal could bleed into the series' future narrative, possibly into Blackbeard's arc and his growing -- but still unknown -- connection to Xebec.
