Australia's Thin "Northern Barrier" in 1941

From Darkest Hour: The True Story of Lark Force at Rabaul - Australia's Worst Military Disaster of World War II, by Bruce Gamble (Zenith, 2006), Kindle Loc. 668-680, 288-295:

To the military planners in Australia, the long string of islands comprising the Mandated Territory of New Guinea and the British-protected Solomons represented a sort of fence. Some in the War Cabinet even referred to it as the “Northern Barrier,” though the islands weren’t fortified until 1941. Lionel Wigmore, an esteemed Australian historian, more accurately described them as “a slender chain of forward observation posts.”

In the fall of 1939, an officer of the Royal Australian Navy (RAN) set out to link the islands with a communications and intelligence network. Over a period of months, Lieutenant Commander Eric A. Feldt traveled “by ship, motor boat, canoe, bicycle, airplane, and boot” from New Guinea all the way to the New Hebrides, single-handedly enrolling dozens of plantation owners, traders, and assorted civilians into a loosely organized group known as the “coastwatchers.” All of them would perform a crucial role the coming war, many at the cost of their lives.

Simultaneously, detachments of a small militia organization, the New Guinea Volunteer Rifles (NGVR), were established among the major islands. Representing the mandated territory’s only infantry force prior to 1941, the NGVR was authorized the day after Australia declared war on Germany, and many of the region’s able-bodied men were volunteers. Lieutenant Colonel John Walstab, the supervisor of police on New Britain, trained a unit of approximately eighty men who formed a rifle company, a machine gun squad, and a small headquarters unit.

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Finally, in early 1941, the AIF decided to send most of the 8th Division to augment the defenses at Singapore, minus the 23rd Brigade, which would garrison three islands north of the mainland: Ambon, Timor, and New Britain. The War Cabinet grandiosely referred to the islands as the “Malay Barrier,” but each small landmass was separated by hundreds of miles of ocean.

The garrisons chosen to defend the islands received operational code names, though none sounded particularly inspiring. Sparrow Force, consisting of the 2/40th Infantry Battalion [= 2nd Battalion of 40th Regiment] plus an antiaircraft battery and troops of the Netherlands East Indies, would be sent to Timor, east of Java. Gull Force, with the 2/21st Infantry Battalion as its nucleus, would fortify Ambon, two hundred miles farther to the north. The last but strategically most important assignment, the defense of Rabaul, went to the 2/22nd Infantry Battalion and its attached units, known collectively as Lark Force.