- “Research battalion or not, the 717th had strong-armed President Aponte into granting permission for its occupation of Montero Vitality Center, and Gao's Honor demanded a response”— Arlo Casille
The Battle of Gao was a battle that took place on Gao over access to forerunner technology.
Sometime in April, 2553, Commander Murtag Nelson discovered the possible presence of a Forerunner Ancilla on Gao. He, along with the 717th battalion took control of the Montero Cave System to search for it. In the following two months, the only thing the battalion had to show for their efforts was the casualties inflicted by the Ancilla's complement of Sentinels. The Ancilla then began to murder civilians in the hopes of forcing the UNSC to withdraw. Meanwhile, the Minister of Protection Arlo Cassile, was unhappy with the occupation of the caverns by the UNSC and the ease with which President Tejo Aponte had given the UNSC permission to stay on the planet, while Venezia was having trade disrupted with the proximity of a UNSC fleet.
On July 2, Cassile convened a meeting between the representative of the Venezian militia, the Keepers of the One Light Reza Lindberk, and Dokab Castor to discuss a way to get the UNSC to leave the Cordoba System. Castor was willing to contribute 500 warriors immediately upon the discovery that the 717th battalion was searching for a Forerunner Ancilla. Casille agreed to smuggle Castor and his warriors into the planet.
On July 4 at 0306 hours, Castor and his first 50 warriors had reached the outskirts of Wendosa Village and had began setting up around the village and also entering the cave system. At approximately 0700 hours Castor observed four SPARTANs loading a body-bag onto a Pelican. Castor, believing it to be containing the Ancilla. ordered his men to attack. Forty snipers fired simultaneously into the village, killing six marines. Afterwards the keepers began to fire mortars into the village while UNSC snipers began their counterattack.
Meanwhile, at the Montero Vitality Center the UNSC was preparing to send a Pelican and two Falcons to provide air-support for Charlie company at Wendosa when the Keepers used their missiles to destroy the Pelican and one of the falcon while almost simultaneously launching a sniper attack.