Forsyth was promoted to acting corporal in March 1917 having been made an acting lance corporal late the previous year. The 3rd Field Company was involved in the Battle of Messines of June 1917, following the infantry and constructing strongpoints in front of the village of Messines once it had been captured. During the following operations around the village of La Basseville, Forsyth was officially noted for his service. From early October, the New Zealand Engineers was involved in the maintenance of the roads during the Battle of Passchendaele, and later that month the 3rd Field Company helped in the consolidation phase of the Battle of Poelcappelle and in sorting out lines of communications and retrieving wounded in the subsequent First Battle of Passchendaele.