Disembarking at Marseilles on 9 June 1916, the 47th Battalion moved into the trenches the following month, where it took part in its first major action of the war at Pozières. For the remainder of the year, Chalmers served alternating stints in the trenches and training behind the frontlines with his battalion. Having contracted trench foot, Chalmers was evacuated to England for treatment in February 1917. During this time, Chalmers married Jessie Alice Courtenay at the register office, Dorset, on 30 July 1917; the pair were later to have a son and a daughter. Having sufficiently recovered, he rejoined the 47th Battalion in France during September.