The Azai clan, local inhabitants of Ōmi Province, were hereditary retainers serving as hikan, or servants, of the Kyōgoku clan, the military governors of Ōmi.  The head of the Azai became a sengoku daimyō with control over the northern district of Ōmi.  Based at Odani Castle in the Azai District of Ōmi, the Azai controlled the surrounding territory.  The Kyōgoku preceded the Azai as sengoku daimyō, governing three districts in northern Ōmi.

After Azai Sukemasa inherited control of the clan, in 1523, turmoil arose within the Kyōgoku clan when Kyōgoku Takakiyo attempted to appoint his second son, Takayoshi, as his successor.  Sukemasa, along with Asami Sadanori, the head of an influential family from Ōmi, supported Takanobu, Takakiyo’s eldest son, as successor, and opposed Takakiyo.  Sukemasa and Sadanori expelled Takakiyo, Takayoshi, and their supporter, Uesaka Nobumitsu, to Owari Province.  Takanobu then became titular head of the Kyōgoku, with the actual power wielded by Sadanori, while Sukemasa performed a key role in efforts to lead protests by the local inhabitants.  Sadanori, however, acted in a tyrannical way, causing Sukemasa to remove him, seize control of the Kyōgoku clan with the support of key retainers, and rise to become a sengoku daimyō and leader of the kokujin movement in northern Ōmi.  

Aiming to expand his territory, Sukemasa opposed Rokkaku Sadayori in southern Ōmi but was pushed back temporarily by the Rokkaku forces.  This caused Sukemasa to enter into an alliance with the Asakura clan in Echizen Province, and with their support, to resist Sadayori and strengthen control over northern Ōmi.

The Rokkaku clan were descendants of the Sasaki and Genji clans of Ōmi.  During this period, the Rokkaku were on the rise, serving to protect the Ashikaga shōgun and participate in the Muromachi bakufu.  It was not to the advantage of Sukemasa to confront the Rokkaku who were former military governors of Ōmi, so he endured frequent incursions through command of the kokujin class in his domain. Meanwhile, Kyōgoku Takanobu became increasingly angry with the despotic nature of Sukemasa, so he reconciled with his father, Takakiyo, and joined with the Uesaka clan and assorted local inhabitants in opposition to Sukemasa.  Sukemasa did not have the resources needed to challenge both the Rokkaku and the Kyōgoku clans, so settled with the Kyōgoku in 1534. 

Years later, in 1541, Takanobu rebelled again, and Sukemasa died in 1542 without a resolution.  It was noted that Sukemasa had intended Taya Akimasa, a son-in-law married to his daughter, Tsuruchiyo, to become his successor.  This led Akimasa to revolt against Sukemasa’s eldest son, Azai Hisamasa, and the ensuing contest for succession triggered serious strife within the clan.  After Hisamasa became the leader, continuing attacks by the Rokkaku caused the clan to gradually lose control.  Following his Coming-of-Age ceremony at the age of fifteen, Hisamasa’s eldest son, Azai Nagamasa, received one of the characters of his original name from Rokkaku Yoshikata as a symbol of subservience of the Azai clan to the Rokkaku, and Nagamasa was compelled to marry a daughter of Hirai Sadatake, a retainer of the Rokkaku, to forge a political alliance between the clans.

In 1559, retainers of the clan became dissatisfied with Hisamasa’s humiliating attempts at diplomacy and backed Nagamasa in a coup d’état against Hisamasa.  He was forcibly removed by the retainers and Nagamasa installed as their new leader based at Odani Castle.  Even after his retirement, Hisamasa continued to have a voice in the political affairs of the clan.  Having scrupulously prepared for the event in advance, in 1560, Nagamasa returned his wife to the Rokkaku and persuaded Takano Bizen-no-kami, lord of Hida Castle in the Aichi District of Ōmi, to betray the Rokkaku and support the Azai.

Furious at hearing this news, Yoshikata attempted to flood Hida Castle. Nagamasa responded by leading a contingent of hard-line retainers to confront the Rokkaku forces across the Uso River.  In the ensuing Battle of Norada, a superior number of Rokkaku troops pushed forward, but just as the men believed victory at hand, an effective counter-assault resulted in an unexpected victory for the Azai. 

To break a stalemate with the Saitō clan of Mino, in 1560, Oda Nobunaga dispatched Fuwa Mitsuharu as a messenger to propose an alliance to Nagamasa.  Although the terms of the proposal were favorable to the Azai, certain retainers, led by Endō Naotsune, opposed the deal.

Around 1567, Nagamasa entered into an alliance with Nobunaga made official by his marriage to Nobunaga’s younger sister, Oichi-no-kata.   Joining with the Oda gave Nagamasa an opportunity to become independent of the Rokakku. Nobunaga marched upon Kyōto to show deference to Ashikaga Yoshiaki, the shōgun, and to deepen his influence in the Kinai through his association with Yoshiaki. Although the alliance with Nobunaga required subservience, it served the intended purpose to hold at bay the Rokkaku.  This arrangement, however, began to unravel in 1570 after the Oda attacked Asakura Yoshikage, a close ally of the Azai.

The conflict between the Oda and the Asakura led to a division of opinion among the senior retainers of the Azai. Based on the insistence of Hisamasa and Akao Kiyotsuna, a veteran commander, Nagamasa annulled the alliance and attacked the Oda forces from behind at the Battle of Kanegasaki.  This marked the end of the alliance and the start of hostilities between the Azai and the Oda.  Just two months after this conflict, the combined forces of the Azai and the Asakura confronted the Oda in the Battle of Anegawa, resulting in a loss for the Azai and the Asakura.

Thereafter, the Miyoshi clan in Settsu Province, along with the Rokkaku, stiffened their resistance toward the Oda.  In 1571, Nobunaga had a falling out with Ashikaga Yoshiaki, whereupon Yoshiaki plotted an encirclement campaign against the Oda by calling upon the Azai and the Asakura and other sponsors in the Kinai Region including leaders of the Hongan Temple.  Yoshiaki further solicited the Takeda clan in distant Kai Province,  but Takeda Shingen died from illness while on an excursion to Mikawa and Owari, favoring Nobunaga in an effort to resist Yoshiaki’s campaign. 

Nobunaga then banished Yoshiaki from Kyōto, and, in the fall of 1573, the Oda attacked the Azai at their home base in an event known as the Siege of Odani Castle.  The Asakura dispatched forces to support the defenders at Odani, but the Oda prevailed.  Nagamasa and Hisamasa committed suicide and the Azai clan perished.