At the end of January, Japanese forces had conquered the north and west coast of Borneo and large parts of Maluku (Moluccas).

On Borneo's east coast, Japanese forces occupied the oil facilities and ports of Balikpapan[1] and Tarakan,[2] and on Celebes the cities of Menado[3] and Kendari[4] had also fallen.

On 1 February, Allied commanders received word from a reconnaissance plane: at Balikpapan, a Japanese invasion force—consisting of 20 troop transport ships, three cruisers and 10 destroyers—was preparing to sail.

[5] It was formed the following day under Schout-bij-nacht (Rear Admiral) Karel Doorman, and began taking on supplies at the Gili Islands, south of Madura.

The ships initially dispersed to deeper water, but the planes left without attacking, and the strike force resumed taking on supplies.

After the hits on Houston and Marblehead, the planes focused on De Ruyter, which evaded four attacks and sustained only minor damage to fire control for its 40 mm guns.

At about 13:00, Doorman ordered his ships to return west and signalled Hart that—without fighter protection—it would not be possible to advance to Makassar Strait, due to the threat from bombers.

The retreat of the strike force resulted in the Japanese taking control of Makassar Strait and thereby tightening their grip on the western part of the Dutch East Indies.