After nine months service in the Victorian Rangers, he enlisted in the
Australian Imperial Force as a private on 9 June 1915. He was commissioned as a second lieutenant in the
37th Battalion in January 1916, was promoted to lieutenant in May 1916, and after training in England, was promoted to captain in France in February 1917. In France he served at
Armentières,
Bois-Grenier,
L'Epinette,
Ploegsteert Wood,
Messines,
La Basse Ville, and
Warneton. He was awarded the Victoria Cross for his actions at Messines. The announcement and accompanying citation for the award was published in a supplement to the
London Gazette of 31 July 1917, reading: Severely wounded in the shoulder by a sniper's bullet, Grieve was evacuated to England, and on recovery returned to his unit in October. However, due to subsequently suffering acute
trench nephritis and double pneumonia, he was invalided to Australia in May 1918. On 7 August, at
Scots Church, Sydney, he married Sister May Isabel Bowman of the Australian Army Nursing Service who had nursed him during his illness. ==Post-war==