The assault on Doniphon was part of the Kwymar Suppressions in 17 BBY. Rebel forces on Doniphon were defeated by the Galactic Empire.
In 17 BBY, the Rebel bases in the Kwymar sector came under attack by the Imperial forces in a campaign known as the Kwymar Suppressions. Doniphon was one of those planets attacked.
X-wings attacked the stormtroopers.
During the battle, many stormtroopers were strafed by Rebel starfighters. One of the injured officers was Sergeant Major Beilert Valance. His platoon was struck by a proton torpedo. Despite those heavy casualties, The Imperial forces were eventually victorious.
After the battle, a badly wounded Beilert Valance was left at Anglebay Station on Telos IV. The medics there saved his life, but only by replacing half of his body with cybernetics.
Years later, Beilert Valance, now turned bounty hunter, attacked and destroyed the Anglebay Station in order to erase all traces of his transformation.
The planet where Valance was injured in Star Wars (1977) 16 was retconned as Doniphon by author Jason Fry in the miniature scenarios The Hunt Within: Valance's Tale. Fry named the planet after Tom Doniphon, John Wayne's character in the 1962 western movie The Man Who Shot Liberty Valance.
In The Essential Guide to Warfare Author's Cut, Valance describes the enemy fighters as "Split-wings. ARC-170s, maybe. Or some kind of Z-95. HUD never painted them with a positive ID." This is likely an attempt to reconcile the depiction of X-Wings in the battle with later media establishing that X-Wings were not produced until circa 1 BBY.