18th BrigadeThe 18th Brigade was formed in October 1939, one of the first three infantry brigades of the Second AIF, and became part of the 6th Australian Division. The designation of the new brigades began at 16 due to the fact that there were already 15 infantry brigades in the Australian Military Forces, not, as is popularly believed, because there had been 15 infantry brigades in the original AIF. The 18th Brigade originally consisted of four battalions: the 2/9th, recruited in Queensland; the 2/10th, recruited in South Australia; the 2/11th recruited in Western Australia; and the 2/12th, recruited in Tasmania and Queensland. In February 1941 the Second AIF was reorganised in accordance with the war establishments of the British Army. This resulted in the reduction of Australian infantry brigades from four to three battalions; shipping arrangements meant that the 2/11th Battalion subsequently became part of the 19th Brigade.
While en route to the Middle East the convoy carrying the 18th Brigade was diverted to the United Kingdom to bolster the forces there following the German conquest of France. This had several repercussions for the organisation of the Second AIF. In the Middle East, the 19th Brigade took the 18th Brigade's position in the 6th Division. In the United Kingdom, the 18th Brigade provided a nucleus for a new brigade, the 25th, and both subsequently formed the basis of the 9th Division when it was raised in October 1940. The 18th Brigade finally arrived in the Middle East in early January 1941. One further reorganisation saw the 18th join the 7th Division in February 1942, where it remained until the end of the war. The brigade fought in the siege of Tobruk, at Milne Bay and Buna in Papua, and at Balikpapan in Borneo.