The Imperial Japanese Navy (IJN) committed 18 submarines in three squadrons to support the operations in the Southern Area. The Southern Area consisted of the Philippines, Singapore, and the Netherlands East Indies. The three squadrons (or flotillas) sent to the Southern Area were the four minelaying submarines of Submarine Squadron 6 (Subron 6) under Rear-Admiral Chimaki Kono, the eight fleet submarines of Subron 4 under Rear-Admiral Etuzo Yoshitomi, and the six fleet submarines of Subron 5 under Rear-Admiral Tadashige Daigo. These submarines were all assigned the same triple mission to
1.) Protect Japanese Army troops wherever they landed,
2.) Patrol the waters off the Malaya Peninsula, and
3.) Attack enemy shipping whenever they appeared.
Many of these submarines were in position to attack Allied naval and shipping operations before the IJN attack on Pearl Harbor, December 7th, 1941.

The following is a listing of significant IJN submarine operations and attacks in support of the conquest of the Netherlands East Indies:

• December 6-7, 1941
I-121 and I-122 laid a minefield off the NE exits from Singapore.
I-54 and I-55 took up patrol positions NE of Kuantan.
I-53 took up a patrol position N of Anamba.
I-57, I-58, I-62, I-64, and I-66 formed a patrol line in the area of Trengganu.

Japanese submarine I-121	Japanese submarine I-122

• December 18-19, 1941
I-123 laid a minefield off Surabaya.

• December 23-24, 1941
I-66 (Cdr. Yoshitome) sank the Dutch submarine K-XVI (Lt. Cdr. L.J. Jarman) while it was returning from attacking Japanese shipping near Kuching.

• January 4-9, 1942
The IJN's Subron 4 (Rear-Admiral Etuzo Yoshitomi) submarines attacked shipping in the Netherlands East Indies.
I-57 (Cdr. Nakashima) sank one ship of 3077 tons and I-56 (Cdr. Ohashi) sank three ships of 7957 tons and torpedoed two more of 5065 tons south of Java.
I-58 (Cdr. Kitamura) sank one ship of 2380 tons in the Java Sea.
I-55, RO-33, and RO-34 are unable to sink any ships in the Anambas area.

Japanese submarine Ro-33

• January 7-13, 1942
The IJN's Subron 5 consisting of the submarines, I-59, I-60, I-62, I-64, I-65, and I-66 covered the landings near Menado and Kema.

• January 9 - February 9, 1942
I-65 (Lt.Cdr. Harada) operated in the Java Sea and then off Rangoon and sank two ships of 6105 tons.
I-66 (Cdr. Yoshitome) operated in the Java Sea and then off Rangoon and sank three ships of 10530 tons
I-59 (Cdr. Yoshimatsu) sank one ship of 4184 tons south of Java and one unidentified ship near Sabang.
I-60 is sunk by the British Royal Navy (RN) HMS Destroyer Jupiter in the Sunda Strait on January 17.

Japanese submarine I-65

Japanese submarine I-60

• January 21-25. 1942
I-55, I-59, I-62, I-64, I-65, and I-66 formed a patrol line in the Ambon Sea to cover the Kendari Landings.

• February 4-18, 1942
I-55 (Cdr. Nakajima) sank one off Java and one ship near Makassar for a total of 6546 tons. The I-55 reported sinking two more ships, but these are unconfirmed.
I-56 (Cdr. Ohashi) sank one ship of 979 tons and another unidentified ship in the Sunda Strait.

• February 5-7, 1942
RO-34 (Lt.Cdr. Ota) encounters a Royal Navy force consisting of the HMS cruiser Exeter, HMS destroyers, Encounter and Jupiter, with the Royal Australian Navy cruiser (RAN) Hobart, NW of the Sunda Strait, but is unable to make a successful attack.
RO-34 also attacked and missed a Dutch Destroyer in the same area.

• February 9-17, 1942
The IJN deployed Subron 4 against the merchant ships loaded with refugees from Singapore, which are proceeding to Sumatra and Java. The merchant ships are escorted by the HMS cruiser Durban, the HMS destroyers Jupiter and Stronghold, and RAN corvettes.
I-55 (Lt. Cdr. Nakajima) sank the ammunition ship Derrymore (4799 tons) N. of the Sunda Strait.
I-56 (Cdr. Ohashi) torpedoed a ship from convoy JS.1 in the Sunda Strait.

Japanese submarine I-55

• February 22 - March 11, 1942
As Java is invaded, once again merchant ships are loaded with refugees now proceeding to Australia. The Imperial Japanese Navy deployed several submarines against this shipping south of the Dutch East Indies.
I-58 (Cdr. Kitamura) sank two ships of 10117 tons and damages another of 6735 tons.
I-53 (Cdr. Nakamura) sank three ships of 11002 tons.
I-54 (Cdr. Kobayashi) sank one ship of 8806 tons.
I-7 (Cdr. Ankyu) sank one ship of 4360 tons.
I-3 (Cdr. Tonozuka) sank a medium sized unidentified ship.
I-4 (Cdr. Nakagawa) sank one ship of 1035 tons.
I-123 laid a minefield in the Torres Strait.

Japanese submarine I-53

Japanese submarine I-58