'Seclusion' was the US geographical rather than operational codename for the Nanpo Shoto island groups between the Japanese home islands and the Mariana islands group in the Pacific Ocean (1941/45).
The Nanpo Shoto, a phrase meaning 'three groups of islands', is the designation of a chain of island groups extending 750 miles (1205 km) to the south of Tokyo Bay as far as a point some 300 miles (485 km) to the north of the Mariana islands group, which can be regarded as a distant extension of the chain. As it name indicates, the Nanpo Shoto comprises of three groups of islands. That farthest to the north is the Izut Shichito with six main islands (Hachijo Shima, Aoga Shima, Bayonnaise, Smith, Mitsugo Jima and Sofu Gan). The central group is the Ogasawara Shoto, which were generally known to the US forces as the Bonin islands group and comprising Muko Jima (Parry), Nishino Shima (Rosario), Chichi Jima (Peel and Beechy) and Haha Jima (Baily, Coffin and Hillsborough), the names in parentheses being common British names. A short distance to the south is the Kazan Retto, commonly known as the Volcano islands group including in its centre Iwo Jima with, to its north Kita (North) Iwo Jima and to its south Minami (South) Iwo Jima. Many documents include Iwo Jima as part of the Bonin islands group (Ogasawara Shoto), but it is in fact part of the Volcano islands group (Kazan Retto). The confusion had occurred because both groups are located in the larger Nanpo Shoto group. It is also a common practice to refer to the two groups together as the Bonin Islands ('Seclusion').
British and US sailors visited the islands throughout the 19th century. The UK claimed the islands in 1827, but did little to maintain its claim, although a small colony of mixed European nationals and Americans was established at Port Lloyd on Chichi Jima. Commodore Matthew C. Perry, on his way to Japan in 1852, recommended that a coaling station be established there. The Japanese then started to colonise the islands in 1853, and in 1861 gained full control of them before annexing all the islands in 1891. The western settlement on Chichi Jima was permitted to remain, but but had largely disappeared by the early part of the 20th century.
Like the Mandated Territories, from the middle of the 1930s the islands were closed to westerners. Military activity in the islands was initially limited, although the Chichi Jima Branch, Army Fortifications Department was formed in 1920 to develop that island’s defences before the Naval Arms Limitations Agreement of 1922 forestalled further fortification. In the late 1930s, the Japanese established a small naval base on Chichi Jima and constructed fortifications. The naval base on Futami Ko harbour on the island’s southern side included a seaplane base, weather and radio stations, and small units of coastal patrol craft. At the start of the Pacific War, the island was garrisoned by almost 4,000 troops and 1,200 naval personnel. A naval airfield was built on Iwo Jima in 1943 and 1,500 airfield service personnel were stationed there, together with an air unit. The garrisons of both islands were increased in 1944 and reorganised into Lieutenant General Hideyoshi Obata’s 109th Division: the parent division was based on Iwo Jima, while Major General Tachibana’s 1st Independent Mixed Brigade and two of the 17th Independent Mixed Regiment's three battalions were located on Chichi Jima.
This part of the division totalled about 25,000 men, of whom many were deployed to the defence of other islands such as the rest of the middle cluster known as Father islands groups (Ani Jima, Ototo Jima and Mago Jima, as well as the Higashi Jima, Nishi Jima and Minami Jima islets). The northern group (Groom islands group) included Yome Jima, Muko Jima and Nakodo Jima. The southern (Mother islands group) comprises Haha Jima, Ane Jima, Imoto Jima and Mei Jima as well as the smaller Hira Shima and Muko Jima. All of these island groups were bypassed when Iwo Jima was chosen for assault, but were subjected to bombing and naval gunfire attack.
The only island in the Nanpo Shoto that would be assaulted was Iwo Jima, whose initial garrison comprised Major General Kotau Osuga’s 2nd Independent Mixed Brigade and Major Tomachi Fujiwara’s 3/17th Independent Mixed Regiment of the 109th Division. Chichi Jima ('Visionary') and Haha Jima ('Mahatma') were considered for seizure, but were rejected because the former was more heavily fortified than Iwo Jima, surrounded by cliffs, and the ruggedly mountainous islands were ill-suited for the construction of the airfields which were a primary US requirement.