This was the Japanese seizure of the northern part of the island of Sumatra, including Sabang on the island of Weh just off Sumatra’s north-western, in the Netherlands East Indies after the modest success of 'L' (i) (12/28 March 1942).
Sumatra is the second largest island in what was then the Netherlands East Indies and the fourth largest island in the world, with an area of 182,812 sq miles (473481 km˛). The Barisan Gebergte mountain chain extends along the entire length of the south-west coast alongside the Indian Ocean, and this varies in height from about 5,000 to 12,000 ft (1525 to 3660 m) with the highest point at Mt Kerinji, to the south-east of Padang, attaining 12,484 ft (3805 m). Between the mountain range and the south-west coast there is only a narrow plain, while lower two-thirds of the island, to the north-east of the mountains, is a broad alluvial plain covered by jungle and bordered by extensive swamps along the coast. The north-western one-third of the island has a much narrower coastal plain to the north-east of the mountains, but this is free of swamps. There are several large rivers draining into the north-east coast, these including the Musi river, which had a dredged channel for small ocean-going ships as far upstream as Palembang, and the Hari river to its north-west, which had a navigable length of 300 miles (485 km).
The largest towns were Medan in the north-west, Palembang and Teleokbetoang in the south-east, and Padang in the centre of the Indian Ocean coast. The port of Sabang, on the small island of Weh just off the north-western tip of Sumatra, was the most westerly port in the Netherlands East Indies. The interior of the lower two-thirds of Sumatra, inland of the coastal swamps, was the primary agricultural region.
Although the island possessed a significant road network, including a road along most of the south-west coast and roads crossing the mountains into the agricultural interior, most of the south-eastern two-thirds of the north-east coast was inaccessible by road. However, there was a road and railway system along the northern one-third of the north-east coast. Another railway system connected Palembang with Teleokbetoang. The overall railway mileage was 1,139 miles (1833 km). There was a regular ferry from Teleokbetoang across the Sunda Strait to Java, and there was also a primitive telephone and telegraph system connecting the major cities.
In 1941, the population of Sumatra and the islands off its coast was in the order of 2.3 million persons, of whom some 6,000 were Europeans and 153,000 Asians and Arabs (predominantly Chinese.)
Sumatra’s jungles concealed great wealth in the form of rich oilfields. Those around the city of Palembang had been in production since 1907 and accounted for half the oil production of the Netherlands East Indies. This oil was especially rich in the valuable lighter petroleum fractions, such as petrol. There was also significant production of coal at Ombilin in the mountains to the north-east of Padang, at Boekit Asam to the south-west of Palembang, and at other places, although most of this was consumed locally. The island also produced rubber and other agricultural products. Economically, therefore, the island was a highly attractive target for resources-poor Japan.
When war broke out in the Pacific, the island had 14 airfields and was defended by 4,500 troops under Major General Roelof T. Overakker, commander of the Middle Sumatra Territorial Command. Though Overakker’s forces constituted the second strongest Dutch troop concentration in the Far East, they were wholly insufficient inadequate to protect an island some 1,000 miles (1600 km) long. The troops were organised into seven battalions scattered along and across the island, with the strongest grouping in the restive north-west rather than around the major prize of Palembang in the south-east.
Palembang was assaulted in 'L' (i) on 14 February 1942 by Japanese paratroops, who seized the oilfields and refinery before the Dutch could undertake significant demolitions. Japanese forces then advanced to take the rest of Sumatra’s south-eastern half, and 'T' (i) was planned for the later seizure of the north-western half to complete the occupation of the entire island, which had been accomplished by 28 March.
Sabang is the primary port on the island of Weh about 9 miles (15 km) off the north-western tip of Sumatra. The island had ab area of about 60.3 sq miles (156.3 km˛), and is ruggedly volcanic. Sabang has an excellent harbour, and by 1941 its facilities included four large cranes and a small 2,600-ton and a medium 5,000-ton dry dock. The port is well located to command the western entrance to Malacca Strait between Malaya and Sumatra.
On 7 March the seaplane tender Sagara Maru and the main elements of the Northern Sumatra Invasion Force earmarked for 'T' (i) departed Seletar on Singapore island and arrived at Batu Pahat, on the coast of Johore later on the same day. The invasion force grouped in the waters to the south of Great Nicobar island. One day later the escort vessel Shimushu also departed Seletar to support the 'T' (i) landings at Sabang, Kota Radja, Idi and Laboehanroekoe, and the minesweepers W-1, W-3, W-4 and W-5 departed Singapore escorting four transports to Sabang and Koetaradja.
Also on 8 March, the 8,407-ton transport Kinugasa Maru departed Singapore with the a troop convoy escorted by the light cruiser Sendai, flying the flag of Rear Admiral Shintaro Hashimoto, under escort of destroyers of the 11th, 12th, 19th and 20th Destroyer Divisions for 'T' (i). The light cruiser Kashii, one of three sister ships designed primarily for the training role, departed Singapore with the light cruiser Yura and the destroyers Amagiri, Asagiri, Ayanami, Isonami, Shirakumo, Uranami and Yugiri, while Vice Admiral Jisaburo Ozawa’s covering force also departed with the heavy cruisers Chokai, Kumano, Mikuma, Mogami and Suzuya, the destroyers Fubuki, Hatsuyuki, Murakumo and Shirakumo of the 11th and 12th Destroyer Divisions, and the seaplane tender Soya Maru.
Another departure from Singapore on the same day was the invasion force in the 4,668-ton transport Heito Maru and 6,344-ton auxiliary minelayer Tatsimiya Maru carrying Major General Takashi Kobayashi’s 'Kobayashi' Detachment of Lieutenant General Takuma Nishimura’s Imperial Guards Division and four army transports (9,256-ton Anyo Maru, 7,378-ton Alaska Maru, 9,418-ton Rakuyo Maru and 6,937-ton Kinugawa Maru) carrying other elements of the same division. The 3,366-ton naval cargo ship Koryu Maru and 1,914-ton water carrier Kisogawa Maru also departed as part of this convoy, but were later detached and headed separately to Penang on the west coast of Malaya.
Cover was provided by the aircraft of the 40th Kokutai from Seletar airfield and of the 'Bihoro' Kokutai from Penang airfield.
On 9 March Shimushu joined the auxiliary minesweepers Reisui Maru and Takao Maru escorting Heito Maru and Tatsumiya Maru, and the heavy cruiser Chokai and light cruiser Yura joined the 1st Escort Unit's light cruiser Kashii and the six destroyers of the 19th and 20th Destroyer Divisions. Later in the same day the seaplane Sagara Maru departed Seletar with units for the Northern Sumatra Invasion Force. On 10 March the minelayer Hatsutaka departed Singapore with the submarine chaser Ch-7 and the 1st Section of the 44th Minesweeper Division as escorts for eight transports including the 6,072-ton Nagoya Maru carrying the main body of the Imperial Guards Division for the invasion of northern Sumatra.
On 11 March the transport ships carrying the Northern Sumatra Invasion Force divided into the Sabang/Idi Group (Tatsumiya Maru, Kinugawa Maru and Heito Maru) and the Koetaradja Group (Anyo Maru, Atlas Maru and Rakuyo Maru).
Early on 12 March the destroyers Amagiri, Asagiri, Ayanami, Isonami, Uranami and Yugiri escorted the ships of the Northern Sumatra Invasion Force, and the seaplane tender Sagara Maru arrived off Sabang to provide air cover for the landings at Sabang on Weh island and at Koetaradja on Sumatra. At the same time the nine transport vessels arrived off Sabang and Koetaradja, and the heavy cruisers Chokai, Kumano, Mikuma, Mogami and Suzuya continued to offer distant cover for the landings, while the 1st Escort Unit's light cruisers Kasii and Yura, and the six destroyers of the 19th and 20th Destroyer Divisions provided close cover.
The six transports of the Northern Sumatra Invasion Force in the Sabang/Idi Group reached their assigned landing areas, and Japanese troops began landing against little or no opposition.
The heavy cruiser Chokai, destroyers Fubuki, Hatsuyuki, Murakumo and Shirakumo, and escort Shimushu supported the Idi landing, and the light cruiser Yura supported the unopposed landing at Laboehanroekoe. The Japanese land forces then moved rapidly to secure the oilfields at Langsa and Pangkalanbrandan, and the seizure of Sumatra was complete by 28 March.