At once brave and impetuous, Nagamasa proved himself an able commander on a tactical level, recapturing land lost to the Rokkaku by his father soon after his appointment. Following a territorial dispute with Oda Nobunaga over Mino province, an alliance was arranged between the two warlords and sealed with the marriage of Nobunaga's sister O-ichi (formerly Shibata Katsuie's wife) to Nagamasa.

In 1570 Nobunaga, now holder of Kyoto, made war against the Asakura of Echizen. Though related to Oda by marriage, Nagamasa was bound by a much longer standing pact of friendship with the Asakura, and came to that clan's aid. In a surprise move he declared against his brother-in-law and threatened the Oda army from the rear even as it drove into the Asakura lands. Nobunaga, assisted by Tokugawa Ieyasu, was able to withdraw more or less without serious loss, but bore Nagamasa a bitter grudge afterwards. Nobunaga and Ieyasu took an army to reduce Odani in the summer of 1570, prompting Asakura Yoshikage to send an army to Nagamasa's aid. The Asai and Asakura lost the resulting Battle of Anegawa (7/70) but replied later that same year with a victory near Otsu that cost Nobunaga a younger brother.

Nagamasa found himself part of a loose anti-Oda confederation that included the unlikely alliance of Asai, Asakura, Rokkaku, and Enryakuji monks, possibly in cooperation with the shôgun Ashikaga Yoshiaki. Further attempts by Nobunaga to bring down Odani were either thwarted by the appearance of the Asakura army or crises elsewhere. Unfortunately, the death of Nobunaga's most formidable enemy, Takeda Shingen, meant that the full attentions of the Oda could be leveled on Ômi and Echizen. In 1573, Nobunaga laid siege to Odani once again, drawing out the Asakura army, which was ambushed and routed before it could reach Nagamasa. Now isolated and seeing the end had come, Nagamasa returned O-ichi and his three daughters to Nobunaga and committed suicide.

The Asai's army was considered an effective force and, in fact, at Anegawa, it came very close to defeating Nobunaga's larger force. The Asai relied on the support of their retainer clans, which included the Isono, Atsuji, Shinjo, Akao, Amemori, and Imai, and in total could muster some 10,000 men for battle. These troops were equipped with a somewhat higher number of rifles then one might find in a clan of the Asai's means since, during the 1560's, they and the Asakura collaborated on a gun-making workshop at Kunimoto (Ômi).

One of Asai's daughters would eventually become Toyotomi Hideyoshi's mistress (the famous Yodo-gimi) and would produce the unfortunate Toyotomi Hideyori, with whom she would die at Osaka Castle (1615). Ironically, another ended up married to Tokugawa Hidetada and was the mother of the 3^rd Tokugawa shogun: Iemitsu.