In 1912, Plant joined the
Australian Army, and was attached to the Administrative and Instructional Staff in
Victoria as a
lieutenant. He would later be assigned to the 15th Light Horse Regiment. Having been promoted to
captain following the Gallipoli landings, he was wounded in the hand on 3 June and left Gallipoli for treatment before returning to the battalion over a month later. Later in the campaign he suffered enteric fever, which necessitated his evacuation to England via Malta. during the brigade's various actions during 1916 and early 1917, including the
Battle of Pozières. For his work, he was awarded the
Distinguished Service Order (DSO). He was awarded a
Bar to his DSO in 1917 for his leadership during the
Second Battle of Bullecourt, in which he rallied straggling infantry under heavy artillery fire. The bar's citation reads: From July, he would serve with the headquarters of
4th Division. When the
war ended in November 1918, he was transferred to the Repatriation and Demobilization Department of the army. For his wartime service, he was awarded the
Croix de guerre and had been
mentioned in despatches a total of five times. He was also made an
Officer of the Order of the British Empire for his service with the 4th Division.
Interwar period Plant, having been made a temporary
lieutenant colonel, eventually returned to Australia in July 1920, along with his wife Oona Hunter Brown, whom he had married in London in early 1918. His service with the AIF ended shortly after his return, and he was reduced to his previous rank and transferred to the
Staff Corps. After a spell with the
Staff College at
Camberley, he held a series of staff posts until 1937, at which time he took up the post of director of military training. On 24 June, Plant, reverting to his previous rank, replaced Brigadier
Alfred Baxter-Cox as commander of the 25th Brigade and led it through the remainder of the
Syrian Campaign. He earned his sixth mention in despatches for his period in command of the brigade. When the
Japanese Empire entered the war, raising fears of a possible Japanese invasion of Australia, he was one of a number of experienced brigadiers recalled to Australia for important positions in the army forces stationed on the
home front. Plant was returned to his temporary rank of major general and appointed commander of the
5th Military District, also referred to as Western Command. This responsibility covered
Western Australia. From April 1942, he was responsible for the
Line of communication in Victoria and then in
New South Wales for the remainder of the war. ==Later life==