[2] A Satsuma Domain samurai initially tasked to study foreign systems for application in the Japanese military, Kawaji fought against forces loyal to the Tokugawa shogunate during the Boshin War.

[2][8][9] His father was a progressive Kanjō-bugyō who negotiated the Treaty of Shimoda, and campaigned for opening Japan with Tsutsui Masanori (1778–1859), a Gaikoku bugyō.

[10][11] Toshiyoshi himself, while serving under Shimazu Hisamitsu, the last Satsuma Domain daimyō, was tasked to study foreign techniques to apply on the Japanese military.

Both being samurai of the Satsuma Domain, Toshiyoshi and Toshiaki played significant roles in the Boshin War and the Meiji Restoration.

[12][13][14][15][16] Before the 1871 abolition of the han system, which effectively removed the daimyōs and bugyōs from their official positions, the new Japanese capital of Tokyo was patrolled by mixed troops of samurai.

[4] Known as rasotsu, Kawaji and Saigō Takamori (a senior samurai also hailing from the Satsuma Domain) were tasked with the recruitment of patrolmen.

[2][6] A total of 2,000 patrolmen initially formed the rasotsu due to Kawaji and Saigo's efforts, and an additional patrolman was recruited for every 3,000 city inhabitants in every prefecture outside Tokyo.

While the primary objective of the mission to renegotiate the unequal treaties was not achieved, Kawaji gathered enough information for him to formulate proposals in reforming the Japanese police, primarily focusing on financing and control structure.

While Kawaji himself did not possess any mastery of the French language, he acquired the services of the interpreter Numa Morikazu, who accompanied him during the Iwakura Mission.

While the Japanese police was later incorporated with German influences, his ideals emphasizing their role in promoting national peace was carried over, and to an extent, contributed to the development of thought control as a state policy in the years leading to the Second World War.

However, it proved difficult to integrate all sword arts, which led to a compromise of ten practice moves (kata) for police training.

The Junsa Kyōshūjo (Patrolman's Training Institute), founded in 1879, provided a curriculum which allowed policemen to study the sword arts during their off-hours (gekiken).

[28] By 23 June, Kawaji, this time promoted as lieutenant general, and his division, had already entered Kagoshima, Saigō's headquarters (around 170 kilometers south of Kumamoto), reinforcing the beleaguered Admiral Kawamura Sumiyoshi and breaking the rebel strength.

[29] The assassination of a high-level government official such as Ōkubo, the last of the Three Great Nobles of the Restoration (the other two being Saigō and Kido), raised concerns with the internal security being provided both by the police and the military.

Considering that Kawaji was also included on the alleged hit list of Ōkubo's assassins, measures were also required to ensure his own safety as the head of Emperor Meiji's security detail.