In 1561, Kenshin and Shingen fought the biggest battle they would fight, the fourth battle of Kawanakajima. Kenshin used an ingenious tactic: a special formation where the soldiers in the front would switch with their comrades in the rear, as those in the frontline became tired or wounded. This allowed the tired soldiers to take a break, while the soldiers who had not seen action would fight on the front lines. This was extremely effective and because of this Kenshin nearly defeated Shingen. In "Kōyō Gunkan" there is one of the most famous instances of single combat in samurai history; during this battle, Kenshin managed to ride up to Shingen and slashed at him with his sword. Shingen fended off the blows with his iron war fan or "tessen". Kenshin failed to finish Shingen off before a Takeda retainer drove him away. Shingen made a counter-attack and the Uesugi army retreated. The result of the fourth battle of Kawanakajima is still uncertain. Many scholars are divided on who the actual victor was, if the battle was actually decisive enough to even declare one, thus is generally considered a draw. It is considered to be the largest casualty battle in the Sengoku period, with loss of estimated 72 percent of Kenshin's army and 62 percent of Shingen's army, but Shingen also lost two of his most important generals during the battle, namely his advisor Yamamoto Kansuke and younger brother Takeda Nobushige. Some more conservative estimates place the casualties around 20 percent.