Morshead subsequently led the 33rd Infantry Battalion through the Battle of Messines in June 1917, and the disastrous Battle of Passchendaele in October, where the 3rd Division suffered heavy losses. In March 1918, the 3rd Division was sent to the Somme sector to help halt the German Spring Offensive. The 9th Infantry Brigade, of which the 33rd Infantry Battalion was a part, was detached from the 3rd Division and sent to Villers-Bretonneux. On 30 March, the 9th Infantry Brigade's commander, Brigadier General Charles Rosenthal, ordered Morshead to restore the front around Aubercourt. With the aid of the British 12th Lancers, the battalion was able to restore the line, although it was unable to advance as far as Rosenthal hoped. The 33rd Infantry Battalion was then relieved, and withdrawn to Villers-Bretonneux to rest. However, on 4 April, Morshead was astonished to find that the line had again been broken and Villers-Bretonneux threatened. In the First Battle of Villers-Bretonneux, the battalion participated in halting the advance, and prevented the Germans from capturing the town. On 18 April, the 33rd Infantry Battalion was still in the town when it was heavily shelled with poison gas. Many men, including Morshead, became mustard gas casualties. He did not return to his unit until June.