Satsuma and Chōshu were not historical enemies during the entire, or most of the Edo period. They had no border disputes, were not allowed to participate in the wider decision making for Japan like most clans, and again like most clans were too busy trying to shore up their own financial situation. Both however were fortunate enough to count themselves among the ones to carry out successful reforms and enter the Bakumatsu from a position of financial strength.

The two clans took different paths following the 1858 Treaty of Amity and Commerce between Japan and the United States, the Ansei Purge, and the assassination of Bakufu Tairō Ii Naosuke in 1860 at the Sakuradamon Incident. After Ii Naosuke's death, the Bakufu decided to try to shore up domestic support by having the young new Shōgun marry an imperial princess. The politicking surrounding this move obviously centered around Kyōto, and many powerful clans who had been normally cut off from the decision-making process in Edo saw an opportunity to gain some political sway by sending men to Kyōto to mostly help lobby for this. Due to this, many powerful men and decision-makers were in Kyōto, shifting the power center slowly, but surely, to Kyōto. After its success, Chōshu opted to support the viewpoint of it's lower-class and frontline samurai activist and threw its weight fully behind sonno jōi, pushing very hard for the repudiation of treaties with foreign powers signed without consent of the Imperial Court and expelling foreigners by force. The sonno jōi supporters from Chōshu, like those from other places in Japan such as Tosa, Mito, and yes Satsuma, were very much of the mind that they should ignore or even go against the Bakufu if necessary (they just assassinated Ii Naosuke after-all) in order to carry out their goals, and were in Kyōto assassinating important people whom they thought disagreed with their view points. However, unlike Chōshu, Satsuma's leadership (and Tosa's and Mito's) were not so hard behind expulsion (and the previous Satsuma daimyō was a foreign-phile) and decided trying to increase their current political influence was more important, going so far as to arrest and kill their own sonno jōi radicals. What Satsuma, and most of these other powerful clans, wanted was a new form of decision making in which the powerful clans gathered in Kyōto would meet to discuss and decide the path of the country. This put the two clans on a collision course.

The chaos on the streets of Kyōto turned Emperor Kōmei and the court, who supported expulsion, against Chōshu, and the emperor gave his support to the Bakufu to keep the peace in the capital. Chōshu's leaders, unaware or willfully ignorant, went so far as to plot to kidnap the emperor, take him to the Kasuga and Ise shrines and make open declarations for jōi. Bakufu forces with Kōmei's blessings moved in to stop this, and in September of 1863 forcibly ousted Chōshu's men from the capital. Satsuma of course supported the Bakufu (and Emperor Kōmei) on this, and in August of 1864 when Chōshu forces attacked the imperial palace in an attempt to kidnap Emperor Kōmei, Satsuma forces played a key role in defending the palace and repulsing the attackers. This is what caused Chōshu's wrath towards Satsuma.