Koga-ryu ("School of Koga"; occasionally spelled "Koka" in English) is an ancient school of ninjutsu. It originated from the region of Koga (modern Koka City in Shiga Prefecture). Members of the Koga school of shinobi (ninja) are trained in disguise, escape, concealment, explosives, medicines and poison; moreover, they are experts in techniques of unarmed combat (Taijutsu) and in the use of various weapons.
The beginnings of the Koka-ryu may be traced to near the end of the Muromachi-era. While the town of Koka was under the jurisdiction of the Rokkaku (lit. "hexagon") clan, it was a kind of autonomous municipality composed of peasant unions, then called so. All important decisions in the municipality were made by a majority vote from the union representatives, this kind of system was uncommon for the period in question.
Sasaki Rokkaku of Omi Province, using Kannonji Castle as a base, started to steadily build up military might. He made light of commands from the Ashikaga shogunate, and eventually began to ignore the shogunate altogether. In 1487, General Ashikaga Yoshihisa brought with him an army to stamp out this rebellion, and a battle between Ashikaga and Rokkaku’s camps ensued. Ashikaga mobilized daimyo from several provinces against the castle of Kannonji, the headquarters of the Rokkaku; as a result, Rokkaku Masayori and Rokkaku Takayori (Masayori's son) were forced to flee to the castle of Kôka. The factual accuracy of their escape is debated and it is likely that they gave up the town to avoid a direct confrontation instead.
Ashikaga then moved his base to Anshiyoji of Kurita District and attacked the castle of Kôka. Kôka fell, but the Rokkaku duo escaped again and ordered the Kôka warriors who followed them to mount a heavy resistance against Ashikaga using guerrilla warfare. Exploiting their geographical advantage in the mountains, the Kôka warriors launched a wide range of surprise attacks against Ashikaga’s forces, and tormented them by using fire and smoke on Ashikaga’s camp during the night. The guerrilla warfare prevented a final showdown, until Ashikaga died in battle in 1489, ending the three-year conflict and sparing the lives of the Rokkaku duo.
The elusive and effective guerrilla warfare used by the Kôka warriors became well known throughout the whole country. This also marked the first time that the ninja of Kôka were drafted as a regular army by their lord. Previously, they were only mercenaries and it was not uncommon to have warriors from Kôka on both sides of a battle. As a result of this victory, the local samurai in the 53 families who participated in this battle were called "the 53 families of Koka".
The last officially known Soke of Koga-ryu was 14th headmaster Fujita Seiko (1898-1966). He taught prior to World War II, and earned such a reputation that he was pulled into teaching for the government during the war. He continued to teach privately after the war, and thus had many students. Some of these students went on to create their own officially recognized systems. Among his students was one Heishichiro Okuse, author of many ninjutsu books and one of the primary references in the 1970 book by Andrew Adams, Ninja: The Invisible Assassins. According to some sources, Okuse served as the 15th Soke of Fujita's line of Koga-ryu, during which time he changed the name to "Iga-ryu" to reflect his residence, and possibly to avoid controversy. That system of ninjutsu is still maintained today by Saito Kazuo, the apparent 16th Soke of Fujita's shinobi lineage.