Nagai Michitoshi served as a bushō during the Sengoku period.  Michitoshi first served as the lord of Takegahana Castle and, later, Kaneyama Castle, in Mino Province.  His common name was Hayato-no-suke.  He was a senior retainer for three generations of the Saitō clan (Saitō Dōsan, Saitō Yoshitatsu, and Saitō Tatsuoki) operating in the central and northern portions of Mino.

Michitoshi was born as a son of Saitō Dōsan, the lord of Inabayama Castle in Dōsan’s early years.  There is a theory that, following the birth of Yoshitatsu, Michitoshi was deemed an illegitimate son of Dōsan or younger brother of Yoshitatsu.  There are also theories that he was either a younger brother of Dōsan, a grandson of Nagai Toshitaka, or son of Nagai Nagahiro given that he inherited the surname of the Nagai family.

Initially, Michitoshi served Dōsan, but as the relationship between Dōsan and Dōsan’s eldest son, Yoshitatsu, gradually deteriorated, in the eleventh month of 1555, he approached Yoshitatsu and proposed the killing of Saitō Magoshirō and Saitō Kiheiji – younger brothers of Yoshitatsu of a different mother who were cherished by Dōsan.  Michitoshi then joined Yoshitatsu in murdering the two youths.  Soon thereafter, in the twelfth month, Michitoshi became a messenger for the allocation of landholdings and stipends by Yoshitatsu.  In the fourth month of 1556, at the Battle of Nagaragawa, Michitoshi and his son, Nagai Michikatsu, fought on the side of Yoshitatsu and defeated Dōsan.  After Yoshitatsu decimated the Akechi clan of the Kani District, Michitoshi became the magistrate for the Akechi manor.

Governance of central and northern Mino Province

In the era of Yoshitatsu, Oda Nobunaga (Dōsan’s son-in-law) of neighboring Owari Province engaged in attacks in the western portion of Mino.  Following the death of Yoshitatsu, Nobunaga expanded into the central portion of Mino and kept the pressure on the Saitō in a conflict known as the Invasion of Chūnō.  There is a letter dated 6/6 from the Zuiryū Temple addressed to the Sekizen Temple stating that Michitoshi and Tatsuoki reconciled, suggesting that Michitoshi was in conflict with senior retainers of Yoshitatsu or his son, Tatsuoki.  A letter dated 6/16 from Takeda Shingen (who brought Shinano Province under his control and allied with Tōyama Naokado of Naegi Castle in eastern Mino) offered reinforcements from Shinano to Michitoshi.  It can be surmised that, in response to a lengthy deployment by Nobunaga in Sunomata in the sixth month of 1561, the forces would join those commanded by Michitoshi and Saitō Tatsuoki.  This can also be interpreted as an offer of support for Michitoshi after a falling out with Tatsuoki in the sixth month of 1560.

In 1561, Endō Morikazu, the lord of Gujō-Hachiman Castle, died and was succeeded by his son, Endō Yoshitaka, at the age of thirteen.  Fearing a crisis, elders in the clan arranged for the marriage of Michitoshi to Morikazu’s widow (Yoshitaka’s mother) and had him serve as a guardian for Yoshitaka.

After the death of Saitō Yoshitatsu on 5/11 of Eiroku 4 (1561), Takenaka Shigeharu and Andō Morinari served Yoshitatsu’s son and heir, Saitō Tatsuoki, who became the head of the family at the age of fourteen.  Tatsuoki’s youth and inexperience shook the band of retainers so that, at once, the Saitō struggled to defend against invasions of Mino by Oda Nobunaga of neighboring Owari Province.  In addition to his immaturity, Tatsuoki indulged in a decadent lifestyle and did not pay attention to affairs of governance.  Furthermore, he took care of only some of his close retainers while distancing Shigeharu and the Western Mino Group of Three (Andō Morinari, Inaba Yoshimichi, and Ujiie Naomoto – influential retainers of the Saitō who later served the Oda) from governance of the clan.

At midday on 2/6 of Eiroku 7 (1564), Shigeharu joined his father-in-law, Morinari, to attack Tatsuoki at Inabayama Castle (later known as Gifu Castle), killing six people including Saitō Hida-no-kami, causing Tatsuoki to flee.  Taking advantage of the seizure of Inabayama Castle, Endō Tanetoshi (the lord of Gujō-Kigo Castle) attacked Hachiman Castle while Endō Yoshitaka fled to Kariyasu Castle.  The occupation of Inabayama Castle continued until around 7/29, and, in the eighth month, Tatsuoki appeared to have taken back the castle by force.

In 1565, Michitoshi sent reinforcements in support of Yoshitaka at Kariyasu Castle and compelled Tanetoshi to return Hachiman Castle.  Michitoshi continued to maintain friendly relations with Takeda Shingen of Kai Province.  On 7/10 of Eiroku 7 (1564), Michitoshi was requested to provide security for Kaisen Jōki on the road to Kai.  This relationship is surmised to have continued until Shingen entered into a political marriage with the Oda (who were in conflict with the Saitō) in 1565.

Based on a sign staked to a ridgepole stating details for construction of the Godo-Hachiman Shrine, ​in the fourth month of 1565, Michitoshi served as a magistrate of Tatsuoki to govern the Kani and Kamo districts including Kaneyama Castle.  There are few traces of Seki Castle, so it is possible that, rather than serving as the lord of Seki Castle, Michitoshi’s main base was at Kaneyama Castle, from which he commanded forces for the Saitō clan in eastern Mino.

Invasion of Mino by Oda Nobunaga

In response to the invasion of Mino by Oda Nobunaga and, in 1565, the assault by Kinoshita Hideyoshi on Unuma Castle on the northern shore of the Kiso River, Michitoshi led 300 soldiers on an attack against Hideyoshi’s forces, but Kinoshita Hidenaga pierced the flank of his contingent, disrupting the attack and enabling Hideyoshi to flee.  Following the betrayal of the Saitō by Satō Tadayoshi (the lord of Kajita Castle), in the ninth month of 1568, Michitoshi planned with Kishi Nobuchika (the lord of Dōhora Castle) to oust Tadayoshi and capture his base at Kajita Castle.  Nobuchika, however, died in the Battle of Dōhora, so Michitoshi holed-up in his base at Seki Castle which then fell in an assault by Saitō Yoshiharu (aligned with the Oda) during the Battle of Seki and Kajita.

Thereafter, Michitoshi joined his stepchild, Endō Yoshitaka to oppose Nobunaga, but, in 1567, after losing Inabayama Castle, he traveled with Saitō Tatsuoki down the Nagara River and fled to Ise Province.

In 1570, Michitoshi participated in the Nagashima Ikkō-ikki and continued the resistance against the Oda.  Thereafter, he served Ashikaga Yoshiaki (the fifteenth shōgun of the Muromachi bakufu) after Yoshiaki’s falling out with Nobunaga.

On 8/28 of Genki 2 (1571), Michitoshi was killed at the Battle of Shiraikawara in Settsu Province.  Under one theory, in 1573, he died along with his lord, Tatsuoki, on Mount Tone at the Battle of Tonezaka (also known as the Siege of Ichijōdani Castle) in Echizen Province.  Alternatively, at the time of Nobunaga’s assault on Inabayama Castle, he may have been expelled or died.

Thereafter, Michitoshi’s sons (Michikatsu, Yoritsugu, and Tokitoshi) changed their surname to Inoue and served Oda Nobunaga and, thereafter, Toyotomi Hideyoshi.