In episode 19 of Special Megabeast Investigator Juspion, the villain Zamurai explains his backstory to our hero during a duel on the beach. The man he fought and lost to, Minamoto no Yoritomo, is an important figure in the history of Japan, having founded the Kamakura Shogunate in 1192.
The battle Zamurai mentions, The Battle of Ishibashiyama, was fought on September 14. 1180 and pitted Yoritomo's Minamoto clan against the forces of Oba Kagechika's Taira clan. The battle was a major defeat for Yoritomo thanks to a darking night raid by the Taira forces and he was forced into retreat. During this retreat was when he met the fictional General who became Zamurai in the show.
Toritomo spent six months gathering more forces before returning to the battlefield. He and his brothers would eventually defeat the Taira at the Battle of Dan-no-ura in 1185. After that, he would cement the supremacy of the Samurai caste in Japan and establish the very first Shogunate in the country's history. This began the feudal era which would not end until the 19th Century. The Kamakura Shogunate itself would fall in 1333 and be replaced by the Ashikaga shogunate.
The above is the symbol of the Kamakura Shoguate.