During World War I, he returned to Britain in July 1916 to enlist in the British Army and was commissioned as a 2nd Lieutenant in the East Lancashire Regiment in December.

[3] On 21 March 1918, between Moyenneville and Ablainzevelle, France, the Germans attacked positions held by Second Lieutenant Horsfall's centre platoon.

Ignoring the wound, he immediately reorganised what remained of his troops and counterattacked to regain his original position.

Despite the severity of his head wound, he refused to go to the dressing station, as the three other officers in his company had been killed.

[4][5] There is a blue plaque commemorating Horsfall on the front wall of Borlase School in Marlow.