The 21st Brigade was a brigade-sized infantry unit of the Australian Army. It was briefly raised in 1912 as a Militia formation providing training as part of the compulsory training scheme. Later, it was re-formed in April 1940 as part of the Second Australian Imperial Force, the unit was raised for service during World War II. As part of the 7th Division the brigade's constituent units were raised from volunteers from several Australian states. After rudimentary training in Australia, the brigade deployed for the Middle East in October 1940. Defensive duties were mounted along the Libyan border in early 1941, before the brigade was committed to the Syria-Lebanon campaign, fighting against Vichy French forces. In early 1942, following Japan's entry into the war, the brigade returned to Australia. After a period of defensive duties in Australia, it was deployed to New Guinea and subsequently played a key role in the Kokoda Track campaign, delaying the Japanese advance towards Port Moresby and then joining the pursuit as the Japanese withdrew towards Buna–Gona. In 1943–1944, the brigade took part in the capture of Lae and the Ramu Valley–Finisterre Range campaign. Its final campaign of the war, came in the final months when it took part in the Balikpapan landings. It was disbanded in 1946.

The battle of Buna–Gona was part of the New Guinea campaign in the Pacific Theatre during World War II. It followed the conclusion of the Kokoda Track campaign and lasted from 16 November 1942 until 22 January 1943. The battle was fought by Australian and United States forces against the Japanese beachheads at Buna, Sanananda and Gona. From these, the Japanese had launched an overland attack on Port Moresby. In light of developments in the Solomon Islands campaign, Japanese forces approaching Port Moresby were ordered to withdraw to and secure these bases on the northern coast. Australian forces maintained contact as the Japanese conducted a well-ordered rearguard action. The Allied objective was to eject the Japanese forces from these positions and deny them their further use. The Japanese forces were skillful, well prepared and resolute in their defence. They had developed a strong network of well-concealed defences.