Aizu(会津,Aizu?) is the westernmost of the three regions of Fukushima Prefecture in Japan, the other two regions being Nakadōri in the central area of the prefecture and Hamadōri in the east. As of October 1, 2010, it had a population of 291,838. The principal city of the area is Aizuwakamatsu.
During the Edo period, Aizu was a feudal domain known as Aizu Domain (会津藩,Aizu-han?). It was part of Mutsu Province; the area once was part of Iwase Province in the 8th century and, before the prefectural system, Iwashiro Province. Although never an official province in its own right, Aizu was considered as such de facto, and even today local Japan Rail stations prefix "Aizu-" to names instead of "Iwashiro-", as it was for stations around the center of Fukushima Prefecture.
The daimyo over much of the Edo period was from the Hoshina family. They had been senior retainers of the Takeda family, and in the early 17th century the head of the family, Hoshina Masamitsu, adopted the illegitimate son of the second Tokugawa shogun Hidetada. As a result, the Hoshina family's fortunes rose, with larger and larger fiefs being given to them, until finally they were moved to Aizu, then rated at 240,000 koku, in the mid-17th century. Hoshina Masayuki, the adopted head of the family, rose in prominence while his half-brother Tokugawa Iemitsu was shogun, and later acted as a regent for his successor, the underage fourth shogun Tokugawa Ietsuna. By the end of the 17th century, the Hoshina family was allowed the use of the Tokugawa hollyhock crest and the Matsudaira surname, and from then on was known as the Aizu-Matsudaira clan, with the name Hoshina being used mainly for internal documents.
In the house code set down by Masayuki, there was a specific injunction to serve the shogun with single-minded devotion, and it was this injunction which the family took great pains to show its adherence to, even if its true objectives were those of improving status and prestige.
Aizu was known for its martial skill, and maintained a standing army of over 5000. It was often deployed to security operations on the northern fringes of the country, as far north as southern Sakhalin. Also, around the time of Commodore Perry's arrival, Aizu had a presence in security operations around Edo Bay.
The domain's two sets of formal rules for its army, the Rules for Commanders (将長禁令 shōchō kinrei) and Rules for Soldiers (士卒禁令 shisotsu kinrei), written in the 1790s, laid down a professional, modern standard for military conduct and operations, including the following two items in the Rules for Soldiers which codified the human rights and protection of enemy noncombatants, over 70 years before the first Geneva Convention of 1864:
Emblem of Aizu domain's infantry at end of Edo period
"Regardless of whether it belongs to the enemy, trampling and ruining rice fields is forbidden."
"In enemy territory, it is forbidden to rape women, harm the elderly and children, desecrate graves, torch the homes of commoners, slaughter livestock needlessly, pillage money and rice, cut trees without reason, and steal crops in the field."
During the tenure of the ninth generation lord Matsudaira Katamori, the domain deployed massive amounts of their troops to Kyoto, where Katamori served as Kyoto Shugoshoku. Operating under the orders of the Shogunate, they also acted as the first official supervisor and patron of the Shinsengumi. Earning the enmity of the Chōshū Domain, and alienating his ally, the Satsuma Domain, Katamori retreated with the shogun Tokugawa Yoshinobu in 1868. Though the Satsuma-Chōshū controlled Imperial Court, following Yoshinobu's resignation, called for the punishment of Katamori and Aizu as "enemies of the Court," he took great pains to beg for mercy, finally acquiescing to calls for war later in 1868, during the Boshin War. Though the Aizu forces fought as part of the greater efforts of the Ōuetsu Reppan Dōmei, they were eventually besieged at Tsuruga Castle, the seat of the Aizu domain, in October 1868.
The Byakkotai ("White Tiger Force"), a group of young, predominantly teenage, samurai, committed seppuku (a form of ritual suicide) on a hillside overlooking the castle after seeing its defences breached.
Yamamoto Kakuma (1828–1892), former samurai, co-founder of Doshisha University.
Takamine Hideo (1854–1910), former samurai, graduate of Oswego Normal School in New York State, Meiji-era educator and head of the Tokyo Normal School, Tokyo Art School, Tokyo Women's Normal School and Tokyo Music School. He is best known for introducing Pestallozian teaching methods to Japan and educational reform.
Ibuka Kajinosuke (1854–1935), former samurai turned Christian pastor, responsible for bringing the YMCA to Japan.
Matsudaira Tsuneo (1877–1949), son of Matsudaira Katamori, ambassador to the U.S. and UK.
Matsudaira Setsuko (1909–1995), daughter of Matsudaira Tsuneo; later married Prince Chichibu no Miya, Emperor Hirohito's brother.
Yamakawa Kenjirō (1854–1931), graduate of Yale University, physicist, researcher, academic administrator, President of Tokyo University and Kyoto University
Yamakawa Sutematsu (1860–1919) graduate of Vassar College, after marriage to Oyama Iwao, she is known as Oyama Sutematsu, an organizer at the Rokumeikan, supporter of numerous organizations such as the Red-Cross in Japan and Women's Patriotic Society. She assisted in the founding of Tsuda College (which was organized by her close lifelong friend Tsuda Umeko)
Yamakawa Hiroshi (1845–1898) Brother of Kenjiro and Sutematsu, a notable military leader who defended the domain, later organized Aizu refugees, a key figure in the relief of Kumamoto Garrison during the Seinan War or Satsuma Rebellion and General in the Meiji Era
Yamakawa Futaba (1844–1909), a co-worker of Takamine Hideo, head administrator at the Tokyo Women's Normal School, she is best known for her support of women's education
Tokugawa Tsunenari (1940 – ), grandson of Matsudaira Tsuneo; current head of the main Tokugawa family.