Within this plan, the 2/21st was earmarked to reinforce Dutch troops on Ambon if the Japanese decided to attack and the battalion subsequently moved to Darwin in the Northern Territory as the likelihood of war with Japan grew.

[3][7] 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.

Meanwhile, Netherlands East Indies forces on the island numbered some 2,600 men, including several companies of Indonesian troops and Dutch coastal artillery.

[8] 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.

[9] 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.

[10] 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.

Outnumbered and lacking air or naval support the 2/21st Battalion, which was guarding Ambon itself, was unable to prevent the advance despite determined resistance, and were pushed to the far west of the peninsula.

[3] In December 2020, one of the battalion's drivers, William Doolan, was posthumously awarded the Medal for Gallantry for his actions around Kudamati village, on Ambon, on 1 February 1942.