Formed in 1914 as part of the all-volunteer Australian Imperial Force from Queensland and Tasmanian recruits, the battalion fought during the Gallipoli Campaign and on the Western Front during the First World War.

During the Second World War the 15th Battalion was mobilised for wartime service and initially undertook defensive duties in Australia before taking part in the fighting against the Japanese in New Guinea and Bougainville in 1943–1945.

In late November, the battalion concentrated at Broadmeadows, Victoria, under the command of Lieutenant Colonel James Cannan, who later rose to become Quartermaster General during World War II.

[4] Following a short period of training, the battalion embarked for overseas on the transport ship SS Ceramic in late December 1914, after marching through the streets of Melbourne.

[7] Travelling via Albany, Western Australia,[1] and then through the Suez Canal, the 4th Brigade disembarked at Alexandria on 3 February 1915 and moved into camp at Heliopolis, where it was assigned to Major General Alexander Godley's New Zealand and Australian Division.

Counter-mining actions were undertaken, but on 29 May a significant attack was put in against the 15th Battalion's position and they were briefly forced back before restoring the situation with a strong counterattack.

[21] Amidst the carnage of the Battle of the Somme which was launched in July, the following month, the 15th Battalion was committed to fighting on the Western Front for the first time, entering the line around Pozières on 5 August, as the 4th Division relieved the shattered 2nd.

[30] After the Allies advanced to follow up the withdrawal, the battalion fought its first major battle of 1917 in early April, around Bullecourt, where the 4th Brigade attacked as a complete formation for the first time since Gallipoli.

After less than 10 hours of fighting, 2,339 soldiers from the 4th lay dead or wounded, out of 3,000 committed,[32] including 400 from the 15th; by the end of the attack, only 52 men from the battalion's assault force remained uninjured.

[33] They were subsequently withdrawn from the line back to farmland around the village of Doulien where the battalion was rebuilt, eventually reaching a strength of 719 personnel of all ranks.

[39] In early 1918, the collapse of the Russian Empire enabled the Germans to transfer a significant number of troops to the Western Front and in March, having amassed 192 divisions, they launched an offensive against the British forces in the Somme.

[40] Heavily outnumbered, the British and Dominion troops were pushed back by the initial onslaught and the Australian Corps was thrown into the line in an effort to stem the tide.

[41][42] The 15th Battalion was moved initially to Bavincourt before securing Hebuterne late in the month, where they experienced a heavy artillery bombardment before turning back a German attack.

Following their arrival, they undertook a support role, constructing defences before moving on to Freschencourt on 22 May, remaining there until they marched at the end of the month to Hamelet near Corbie, where they conducted several patrol actions.

[45] That month, a number of American troops were assigned to the battalion to gain experience, as the United States began building up its forces on the Western Front.

[46] Lieutenant General John Monash, having taken over as commander of the Australian Corps from William Birdwood, decided to launch a combined arms attack to reduce the salient that had developed in front of Amiens around Hamel and straighten the line.

Coming up against strong resistance from well-sited machine-gun posts, the advance was held up until they were overcome by Lewis gun teams and section-level fire and movement, which allowed the battalion to gain the Pear Trench.

The 15th Battalion's sole Victoria Cross recipient, Private Henry Dalziel, received his award for his actions during this time while serving as a "number two" within a Lewis-gun team, rushing a German machine-gun post with a revolver.

They did not return to action before the armistice was signed in November 1918,[60] after which the battalion's strength was slowly reduced as men were individually repatriated back to Australia as part of the demobilisation process.

[69] This, combined with economic pressure caused by the Great Depression, which made it difficult for part-time soldiers to maintain their livelihoods while continuing their training commitments, meant that the number of volunteers fell sharply.

[65] This formation was split in July 1939,[71] as part of a hasty expansion of Australia's military force as tensions in Europe raised concerns about the country's preparedness in the event of a future war.

[78] Following Japanese victories in Malaya, Singapore and elsewhere in the Pacific in late 1941 and early 1942, the whole 15th Battalion was mobilised for wartime service and tasked with undertaking defensive duties along the coast of south-east Queensland, based at Caloundra, due to concerns of a potential invasion.

[79] To be classified as an AIF battalion, under the provisions of the newly passed Defence (Citizen Military Forces) Act 1943, 65 per cent or more of the personnel had to volunteer for service outside Australian territory.

[87] Following the fall of the town, as the remainder of the 5th Division was rested, the 15th Battalion, which had seen less action due to being held in reserve, was committed to pursuing the withdrawing Japanese.

[88] The battalion advanced north towards the Markham River, and on 14 September two companies were transported by barge in an effort to get behind the fugitives and cut them off, but they arrived about an hour too late.

[91] The battalion subsequently took up defensive duties around the town as the 29th Brigade was assigned the task of holding Lae to free up troops from the 7th Division for operations in the Finisterre Range further to the west.

[101] The 15th Battalion occupied a position around Sisikatekori and along Killen's Track, in the southern sector of the brigade's area, while the Australians made preparations to resume the advance towards the Buin stronghold.

The most-forward battalion, on 6 July it was subjected to heavy attacks as the Japanese sought to infiltrate the Australian forward positions and cut their lines of communication.

[101] After this, patrols were sent out across the Mivo, but strong Japanese resistance, and heavy rain,[103] frustrated the 15th's efforts to carry out a detailed reconnaissance of the opposite bank, effectively halting the Australian advance and ending significant combat in the island's south.

[104] The advance was cancelled a week later in anticipation of a Japanese surrender, following news of the atomic bombing of Hiroshima and Nagasaki, and the battalion's involvement in the fighting on Bougainville came to an end.