The 2/9th Battalion was an infantry battalion of the Australian Army during World War II. Raised in Queensland as part of the Second Australian Imperial Force shortly after the outbreak of the war, it formed part of the 18th Brigade and over the course of the war it was attached to the 6th, 9th and 7th Divisions due to several re-organisations. It served in the United Kingdom in 1940, forming part of a small Australian garrison sent there to help defend against a possible German invasion, before being transferred to North Africa where it took part in the Siege of Tobruk and then undertook garrison duties in Syria following the Syria–Lebanon campaign in 1941.

The 18th Brigade was an infantry brigade of the Australian Army. The brigade briefly existed as a Militia formation prior to the First World War, but this was short-lived. During the Second World War, the brigade was raised on 13 October 1939 and was one of the first three infantry brigades of the Second Australian Imperial Force to be formed. Initially commanded by Brigadier Leslie Morshead, it served in the United Kingdom in 1940–1941, where it helped bolster the British garrison in anticipation of a possible German invasion following the Fall of France. In early 1941, the brigade was transferred to the Middle East where it later took part in fighting against the Italians in Libya and then helped to defend the besieged port of Tobruk before fighting against the Vichy French in the Syria–Lebanon campaign. The 18th Brigade was withdrawn to Australia in early 1942, and it later took part in the fighting against the Japanese in Pacific fighting several campaigns in New Guinea between late 1942 and early 1944. Its final involvement of the war came in mid-1945 when it took part in re-taking Balikpapan. Following the end of hostilities, the 18th Brigade was disbanded on 3 January 1946.