However, Free Dutch forces—mainly the Royal Netherlands Navy and the 85,000-strong Royal Netherlands East Indies Army (KNIL, including a small air service)—fought on, spread throughout the Dutch East Indies, and by December 1941 under an embryonic and somewhat chaotic joint Allied command which became the short-lived American-British-Dutch-Australian Command (ABDACOM).

With its rich petroleum exploitation capacity, for instance at Tarakan, Balikpapan and Banjarmasin, Borneo was a prime target for Japan, and a very poorly guarded one.

Chronically short of natural resources, Japan needed an assured supply of fuel to achieve its long-term goal of becoming the major power in the Pacific.

Furthermore, the Brunei Volunteer Rifles, a small local force of 160 men organised by the British administration, supplemented the region's defences.

After having heard of the attack on Pearl Harbor, on 8 December 1941, the Brooke government ordered that the oilfields at Miri and Seria and refinery at Lutong be quickly demolished.

On 25 November, five Brewster 339 Buffalo fighter planes arrived for local defence, followed in the beginning of December by Martin B-10 bombers.

The Dutch Naval Aviation Group GVT-1, with three Dornier Do 24K flying boats, was located in Pontianak along with a KNIL garrison, commanded by Lieutenant Colonel Dominicus Mars, numbering approximately 500 men.

Dutch forces in West Borneo consisted of the following units: The main Japanese force—led by Major General Kiyotake Kawaguchi—consisted of units from Canton, southern China: On 13 December 1941, a Japanese invasion convoy left Cam Ranh Bay in French Indochina, escorted by the cruiser Yura (Rear-Admiral Shintaro Hashimoto), the destroyers of the 12th Destroyer Division (Murakumo, Shinonome, Shirakumo and Usugumo), the submarine chaser CH-7, and the seaplane tender Kamikawa Maru.

The convoy escaped detection and, at dawn on 16 December 1941, two landing units secured Miri and Seria, encountering very little resistance from British forces.

[8] On 16 December 1941, 9 days after the Pearl Harbor bombing, Japanese troops consisting of 10,000 men landed on Tanjung Lobang Beach, Miri without much resistance.

[8] However, on 17 December 1941, a Dutch flying boat from Tarakan Island attacked the Japanese destroyer Shinonome (under the command of Hiroshi Sasagawa) off Miri.

[8] Dutch Martin B-10 bombers attacked Japanese shipping from their base, 'Singkawang II' at Miri, on 17 December, but their attempt failed.

One flying boat scored two 200 kg bomb hits on Shinonome, causing a massive explosion, while a near miss ruptured its hull plating.

On the night of 23–24 December, HNLMS K XVI torpedoed the Japanese destroyer Sagiri 30 miles (48 km) north of Kuching, becoming the first Allied submarine in the Pacific to sink a warship.

[10] Meanwhile, on 31 December 1941, the force under Lieutenant Colonel Watanabe moved northward to occupy Brunei, Labuan Island, and Jesselton (now called Kota Kinabalu).

On 18 January 1942, using small fishing boats, the Japanese landed at Sandakan, the seat of government of British North Borneo.