During the
First World War, Holmes served first as Commander of the
Australian Naval and Military Expeditionary Force (AN&MEF), and later as Commander of the
5th Brigade, which he took to Gallipoli and the Western Front.
German New Guinea Shortly after war broke out, a special expeditionary force, the AN&MEF, was formed to occupy German possessions in
New Guinea and the
Pacific Islands. Holmes was appointed the commander. Under Holmes command the force sailed from Sydney on the auxiliary cruiser on 19 August 1914. The German governor surrendered to Holmes on 12 September 1914 and the
British flag was raised over the town. made a habit of regularly visiting every key part of his front lines. When Major General
James Legge fell ill in November and was evacuated, Holmes took over acting command of the 2nd Division. He was still in temporary command of the division in December 1915, when they were amongst the last of the Anzac troops to be evacuated from Gallipoli. replacing Major General
Vaughan Cox. Holmes continued his habit of personally reconnoitring every part of his line. Holmes commanded his division at Bullecourt in April, where he opposed the operation that ultimately caused his division very heavy losses, and at
Messines in June. On 2 July 1917, Holmes took the
Premier of New South Wales,
William Holman, to survey the Messines battlefield. A memorial service was held for him in October 1917 at St Matthias' Anglican Church, Paddington, the church where he and Susan were married. ==Memorial==