Niwa clan
Nagakuni was born in Nihonmatsu as the 6th son of Niwa Nagatomi. He succeeded to the family headship upon his father's retirement in 1858. He was ordered by the Tokugawa shogunate to send military forces to increase security in Kyoto in 1864 and 1865. In 1868, with the start of the Boshin War, Nihonmatsu joined the Ōuetsu Reppan Dōmei, but was defeated by the forces of the Satchō Alliance, with Nihonmatsu Castle burning down during the Battle of Nihonmatsu on September 15, 1868. He subsequently made peace with the Meiji government, which ordered him to retire, and which reduced Nihonmatsu Domain from its previous "kokudaka" of 100,700 "koku" to 50,700 "koku". After the deaths of his son and grandson, he reassumed the chieftainship of the Niwa clan in 1902, and also the "kazoku" title of "shishaku" (viscount). He died in 1904 and his grave is at Aoyama Cemetery in Tokyo.