The 'Battle of Lamon Bay' was a secondary undertaking in the Japanese invasion of Lingayen Gulf during the Japanese 'M' (ii) conquest of the Philippine islands group (21/23 December 1941).

The seizure of the area of Luzon island’s coast to the south-east of Manila was designed to complete the Japanese encirclement of the Filipino capital and to also to create a diversion for the Japanese main invasion force from Lingayen Gulf to the the north of Manila. Lamon Bay is a large bay on the eastern coast of Luzon, to the south-south of Manila, but the prevailing winds of December rendered it a poor landing site that was also isolated from Manila by the Tayabas isthmus.

The Lamon Bay invasion force was led by Lieutenant General Susumu Morioka, and comprised 7,000 men of Morioka’s 16^th Division. The divisional commander planned a three-pronged landing at Mauban, Atimonan and Siain, from which his forces were to advance immediately after landing along Route 1 toward Laguna de Bay without waiting to secure the narrow landing site.

The invasion force departed Amami Oshima in the Ryukyu islands group, between south-western Japan and Formosa, on 17 December in 24 transport vessels only six hours after the Lingayen Bay invasion force had sailed from Formosa. The Lamon Bay force was initially escorted only by four destroyers and four minesweepers, but en route was joined by Rear Admiral Kyuji Kubo with one light cruiser, two destroyers, two minesweepers and one minelayer from the Legazpi operation. On 23 December, the convoy was sighted by the submarine Sculpin, but this US boat was unable to cause any damage. On the morning of 24 December the convoy reached Lamon Bay.

The US and Filipino opposition to these landings was highly disorganised. The area was nominally under the control of Major General George M. Parker’s South Luzon Force with the Philippine army’s 41^st Division and part of its 51^st Division. Much of the remainder of the 51^st Division had been moved south to counter the Japanese landing at Legazpi. Other troops were still in the process of moving into position when the Japanese landed, and suffered from lack of artillery. What little artillery the South Luzon Force possessed was located on the western coast, and MacArthur refused Parker’s request for additional artillery several times.

The Lamon Bay invasion force comprised the light cruiser Nagara, the heavy cruiser Ashigara, the destroyers Kawakaze, Suzukaze, Tokitsukaze, Umikaze, Yamakaze and Yukikaze, the minelayer Aotaka, the minesweepers W-7 and W-8, the gunboat/minelayer Ikushima Maru, the gunboats Busho Maru, Keiko Maru, Kanko Maru and Myoken Maru, the submarine chasers Shonan Maru No. 17 and Takunan Maru No. 5, the netlayer Fukuei Maru No. 15, the Imperial Japanese army transport vessels Bengal Maru, Dainichi Maru, Durban Maru, Kaimei Maru, Kayo Maru, Kitano Maru, Kofuku Maru, Lisbon Maru, Nagato Maru, Nichiren Maru, Ryoka Maru, Ryuyo Maru, Shinsei Maru, Shinshu Maru, Taian Maru, Tamon Maru No. 5, Tatsuno Maru, Tofuku Maru, Toyama Maru and Toyohashi Maru, and the Imperial Japanese navy transport vessels Hakusan Maru, Kimishima Maru, Myoko Maru and Senko Maru.

The first Japanese detachment to land was the 2/20^th Regiment under the command of Lieutenant Colonel Nariyoshi Tsunehiro. Coming ashore at Mauban at the break of day on 24 December under cover by aircraft provided by the seaplane carrier Mizuho, the Japanese immediately came under attack by the Philippine 1^st Regiment, which was dug in on the beach. US aircraft also attacked the transport convoy. However, by 08.00 the Philippine army had been pushed back from the beach and by 08.30 Mauban had been secured by the Japanese. The Philippine army fell back 5 miles (8 km) to the west, halting the Japanese advance towards Lucban and the southern shore of Laguna de Bay at 14.30.

The second detachment to come ashore was the 2/20^th Regiment, which landed without opposition at Sianin to cover the left flank of the main invasion force, and to act as a reserve. One company was sent along the railway to Tayabas Bay and the main force moved to the south-east to link with the 'Kimura' Detachment, which was advancing to the north-west from Legazpi. The columns met on 27 December, thereby cutting off the escape route of the survivors of the Philippine 51^st Division, who were still retreating from Legazpi.

Coming ashore 2 miles (3.2 km) to the south of Atimonan, Morioka’s main force comprised the bulk of the 20^th Regiment, the 16^th Reconnaissance Regiment and 22^nd Field Artillery Regiment. The 20^th Regiment had occupied Atimonan by 11.00 despite stubborn resistance by the Philippine army, while the 16^th Reconnaissance Regiment bypassed the town and pushed across the mountains to Malicbuy, where the Philippine 2/52^nd Regiment was still in the process of setting up defences. With assistance of bombers of the Imperial Japanese army air force’s 8^th Sentai, the 16^th Reconnaissance Regiment quickly overwhelmed the defenders.

The next US defensive line was set up along a river near Binahaan, about 4 miles (6.4 km) to the west of Malicbuy. However, by late afternoon, Morioka’s main forces had completed mopping-up operations in Atimonan, and the Japanese were thus able to bring their full force against the US position at Binahaan. The Americans retreated under cover of darkness along Route 1 to Pagbilao, with the Japanese in pursuit.

By the evening of 24 December, the Japanese had successfully landed at Lamon Bay, and at the cost of only 84 men killed and 184 wounded had completed its encirclement of the approaches to Manila from the south. To the north, the Lingayen Gulf invasion forces had similarly achieved its objectives in securing the northern and eastern approaches to Manila. Lieutenant General Masaharu Homma, commander of the 14^th Army, who had hitherto possessed a poor opinion of the quality of the 16^th Division as a result of its poor combat record in China, now expressed considerable surprise at its success. By the evening of 24 December, Homma had transferred his staff ashore at Bauang, where he established the headquarters of his 14^th Army in preparation for the final drive on Manila.