On the Western Front Wallach saw action at the Battle of Pozières in August 1916. He named the trench he occupied at Pozières "Blancmange Trench" because it changed shape every time he visited it. In 1918, the 19th Battalion helped to repel the German spring offensive, and it was during this time, on 7 April 1918, that Wallach, by now a captain and commander of a company, was involved in an attack around Hangard Wood.