The God Valley Incident took place at an island known as God Valley, 38 years before present time. It consisted of a battle between the Rocks Pirates and a joint effort of the Roger Pirates and the Marines.
For unknown reasons, the world nobles and some of their slaves were present at God Valley during this incident. In order to protect them from Rocks D. Xebec and his crew, who were planning an attack on the nobles, current Marine vice-admiral Monkey D. Garp had to form a temporary alliance with Gol D. Roger, captain of the Roger Pirates.
The Rocks Pirates were eventually defeated by the Pirate-Marine alliance, leading to Rocks' death and the crew's disbandment.
The results of this particular event would be the ones to brand Garp with massive recognition and the title of "Hero of the Marines".
Roger's involvement itself, however, was covered up by the World Government. The same applied to many of Rocks' activities as the captain possessed the Will of D. and planned to oust the world nobles to become ruler of the world himself, meaning only very few Marines, mostly veteran ones, know about the details of this incident.
The island of God Valley also ended up mysteriously vanishing, leaving no trace of its existence behind and no longer showing up in maps of the world.