On 10 April Jackson's Division took over a forward position in the eastern Amentieres section of the Western Front. As a prelude to what became known as the Battle of the Somme, orders were issued for raids to be carried out on enemy positions between 20 and 30 June 1916.
On the night of 25 June, Jackson was acting as a scout for a party of forty soldiers, as they carried out an assault on the forward trenches of a Prussian infantry regiment, south-east of Bois Grenier (near Armentières). During the assault Jackson captured an enemy soldier and returned with him through no man's land. Prisoners were valued for the purpose of interrogation. On learning that some of his party had been hit in the intense shelling and gun-fire, Jackson returned to no man's land. He helped to bring in a wounded man, before going out again. While assisting Sergeant Camden to bring in the seriously wounded Private Robinson, a shell exploded nearby. The blast rendered Camden unconscious, blew off Jackson's right arm above the elbow and inflicted further wounds to Robinson.
Despite the loss of his arm, Jackson managed to return to his trenches, claiming he only felt "a numbing sensation". An officer applied a tourniquet to his arm, using a piece of string and a stick, and Jackson returned to no man's land for another half an hour until he was satisfied there were no wounded men left on the battlefield.