The Japanese launched an invasion on several small islands and cities in the Philippines just hours after Pearl Harbor. They had been taking more and more towns and islands daily, and encountered only minor resistance, mostly by air forces. After a minor skirmish on Naga on 17–18 December, the Japanese headed towards Sipico.
Pushing north from Naga, the Japanese reached Sipoco on 19 December 1941. On 21 December, the Japanese decided to attack. The Filipino troops were positioned well, and managed to successfully drive off the Japanese, inflicting serious casualties, while taking minor losses themselves. On 23 December, the troops were ordered to leave the city when a Japanese invasion force appeared off Atimonan. Part of the retreating force was cut off, but some managed to escape.