The outbreak of the First World War in August 1914 meant that the light horse as an idea and organisation, although not the militia units already in existence, was to face its test in war. The last despatch of Australian mounted troops to a war had been a mixed experience, but since then Australia's mounted branch had undergone significant change. Brought together under a single national scheme, it had trained accordingly and, even if there had been significant problems, matters had clearly improved overall. The militia light horse bequeathed to the regiments of the Australian Imperial Force (AIF) its organisational template, role and many of its men. The demands of war, however, meant that it soon developed a competence and efficiency far beyond what the militia had ever come close to achieving. By 1917 light horse regiments had fought in three theatres in which the British Empire was engaged and in one of them, Egypt, became the centrepiece of the force that campaigned against the Turks in the second half of 1916.

Formation to Gallipoli

The Defence Act, which had frustrated Hutton by forbidding the sending of Australia's militiamen overseas, meant that on the outbreak of war attention quickly turned to the raising and despatch of a separate expeditionary force composed entirely of volunteers. Accordingly, on 10 August 1914, the new AIF commenced recruiting to a plan set out by the then inspector-general, and soon to be AIF commander, Brigadier-General William Throsby Bridges. Reflecting the existing military structures, the forces raised by the Commonwealth Government were meant to be roughly commensurate with the existing militia contribution of each state to fulfil the initial offer of an infantry division and a brigade of light horse, both with the necessary support elements. Half the rank and file was to be recruited from the militia, the remainder from men who had previous war or militia service but were not presently serving.