In 1575, when Nobunaga learned that Nagashino Castle was besieged by Takeda Katsuyori's large force, he marched with Ieyasu and 38,000 allied troops to relieve it. The brave defense by Okudaira Sadamasa bought time for the relief force, and Nobunaga's musket tactics at Shitaragahara destroyed the Takeda army. The relief of Nagashino was a landmark demonstration of Nobunaga's military innovation.

In 1575, when Takeda Katsuyori besieged Nagashino Castle, Ieyasu joined forces with Oda Nobunaga to relieve the siege. On May 21, their combined army crushed the Takeda forces at the Battle of Nagashino (Shitaragahara), rescuing the garrison under Okudaira Nobumasa. This decisive victory consolidated Ieyasu's hold over Mikawa and Tōtōmi provinces.