The beginnings of an alliance between the two domains might be seen in agreements made in 1859-1860, as Shimazu Hisamitsu, regent to the young Satsuma daimyô Shimazu Tadayoshi, pursued the expansion of Satsuma's domestic trading partners. Satsuma established a trading office in Shimonoseki in 1859, sending two trading ships to the port at that same time, exchanging Ryukyuan textiles, sulfur, and indigo for whalebones, salt, and cotton from Chôshû. The following year, Chôshû sent a prominent domain merchant along with a samurai official to Satsuma to negotiate further trade relations. For the next couple of years, the two domains enjoyed a lively trade in sugar from Ryûkyû and Amami in exchange for salt, whalebone, and rice from Chôshû.
However, the relationship soon turned sour. In accordance with Emperor Kômei's imperial edict ordering the violent expulsion of the barbarians, Chôshû began on 1863/5/10 to fire upon passing ships. In 1864/2, Chôshû's coastal batteries hit a merchant vessel sponsored by Satsuma carrying a cargo of cotton to sell to Western merchants at Nagasaki, leading to considerable tensions between the two domains, and Satsuma severing trade relations with Chôshû. Later that year, Chôshû sent an armed force to Kyoto with intentions of enhancing the domain's political influence in the city; when that force attempted to storm the Imperial Palace, warriors from Satsuma and Aizu han repelled them. The Court subsequently declared Chôshû an enemy of the state, and Satsuma warriors took part in a punitive expedition against Chôshû launched by the shogunate.
The two domains reached a rapprochement, and in 1866/1, Saigô Takamori and Katsura Kogorô met at Komatsu Tatewaki's residence in Kyoto. Komatsu and Sakamoto Ryôma served as mediators, as the group laid the groundwork for the Alliance, signing a secret pact to work together to overthrow the Tokugawa shogunate. Trade relations between the two domains were also restarted; whereas previously Chôshû had left the details of the trade up to local merchants, the domain now assigned officials to oversee the arrangements.
- Robert Hellyer, Defining Engagement, Harvard University Press (2009), 196.
- Hellyer, 187.
- Plaques on-site at former site of Imaizumi Shimazu family mansion, Dairyû-chô, Kagoshima.
- Hellyer, 202.