The battalion began arriving in Darwin on 9 April 1941 and spent the next nine months training and on garrison duties; however, the amenities were isolated and uncomfortable and preparations were hampered through a lack of equipment. Following the Japanese invasion of Malaya on 8 December the battalion prepared to move, arriving on Ambon on 17 December as part of Gull Force. Under Roach's command Gull Force consisted of the 2/21st Battalion supported by anti-tank artillery, engineers and other supporting arms with a combined strength of 1,100 men. Meanwhile, Netherlands East Indies forces on the island numbered some 2,600 men, including several companies of Indonesian troops and Dutch coastal artillery. These troops were tasked with defending the Bay of Ambon and the airfields at Laha and Liang which were being used by a small number of Dutch and some Australian aircraft from No. 13 Squadron RAAF. Yet with the small Australian and Dutch force totaling just 3,700 men, Roach believed Ambon unable to be defended with the limited military resources available and he urgently requested reinforcement. He was subsequently relieved of his command, and was replaced by Lieutenant Colonel John Scott on 17 January 1942. Just prior to the Japanese landings Scott altered the location of many of his defensive positions, which resulted in the battalion being less prepared to repel the invasion.