The letter was written by Tokugawa Nariaki (1800-1860), a daimyo who ruled the Mito Domain (present-day Ibaraki Prefecture). It was found among 103 letters and documents from leading figures during the waning days of Japan's feudal past and after the Meiji Restoration of 1868. They are kept by the Nozaki family, famed as the "king of salt farms of Kojima" during the Edo Period (1603-1867). Word of the trove came from the Ryuo Kaikan Foundation, which maintains and manages the old Nozaki family residence, a government-designated important cultural property here. Foundation officials said they have been analyzing the documents since 2012, adding that many materials represented new discoveries. Of them, 16 documents are related to Nariaki, a prominent nationalist and the father of Tokugawa Yoshinobu, the 15th and last ruler of the Tokugawa Shogunate. One letter begins with a subject line in red ink that states, "About burning barbarians to death." It was sent to his close aide Fujita Toko in 1855, the year after the Japan-U.S. Treaty of Peace and Amity was signed. Nariaki displays his bitter resentment toward the United States, which in his view disrespectfully sent warships and demanded that Japan open up its ports under threat of an attack. He discloses his plot to board one of the vessels under the pretext of wining and dining U.S. delegates, seize the ship and cut down high-ranking members with swords. According to the foundation, the wife of the top official in charge of an administrative district that included the Kojima area was a daughter of a samurai from the Mito Domain. This explains how the Nozaki family ended up with a large number of documents from the Mito Domain.