Perhaps you where mistakingly thinking of Kanyū, it's a general term for Japanese Martial Arts that refers to the old Japanese adaptations of martial arts.
Martial arts, not Fighting or Soldiers, but the generalized education, improvement, adaptations, and passing that knowledge forward started around 4000b.c. In India, carrying with it Hindu & Buddhist influences. It traveled through China, then Korea, where it crossed the korean peninsulaarriving in Japan around the 4th century. Each country had their own variations or adaptations, initial adaptations followed by evolution over time.
Look at Mantis style Kung Fu in China, the Northern Chaplin and southern Shaolin differences was a result of genetics. The Average height and reach of someone from southern China was shorter, than their Northern brethren, most likely due to the Mongolian cohabitation pre Great Wall Imperial China. So southern Mantis was more grappling, traits of the actual preying Mantis kill stage. However in the North the longer reach allowed for more high velocity striking at a distance. They both made modifications to benefit their capabilities.
In Korea the greatest threat was the soldiers from the Khan dynasty on horseback. That caused the increased skill set development of strong kicks delivered in the air, a skill that allowed Korean Warriors to kick the riders off of their horses, once on equal ground strikes found in TKD as well as disarming and controlling the enemy through Hapkido. At the time they were probably only early versions of both Korean martial arts, but you can understand. From there we enter Japan, Korean influence transfers kumdo into Kendo, Hapkido into judo, TKD into Karate, again using more modern styles to present a basic understanding.
After British Colonization in Asia hit it's highest point, the Japanese and Chinese martial arts where positioned to test each other for superiority. Japan and Korea where too similar in the eyes of the British, so it wasn't perused. But the styles and effectiveness between Japan and China where vastly different by that point. The Chinese where trained in a dramatic range of styles from wish I, wing chun, two chi, All the shaolin styles, plus their larger level of interaction with southeast Asian, Middle Eastern, Mongolian, & European travelers created a huge amount of weapon influences and reverse engineering them to a degree to benefit the training with original Chinese designs and combine them for maximum user familiarity and some added perks.
However, Japan had large adaptations that arose from the geography, independent clan interaction, and a feudal period that had caused internal conflict, which created an environment where perfection of core skills & the military leaders who where primarily lineage based, made alterations in every military variable in the goal of surpassing the opposition.
Take the Shinobi, initially limited to clans in Iga, who lived in the mountains. They utilized farming tools, to create weapons that could have diverse functionality. Th e potato fork, that could also be used to chip away at castle mortar, to remove stones and gain entry. Modern belief of the sword swinging ninja assassins are wildly exaggerated. They weakened military powers through intelligence gathering, and politics. The enemy of my enemy is a friend, plus while we work together we can gain a large amount of intelligence during the alliance.
They sought intelligence and it was primarily utilized to uphold peace. They where masters of adaptation and espionage, but the Shinobi weren't classified as a martial art, not until much more recently. It was a military strategy, incorporated with existing Martial art styles, and a Shinto and Mikkyo both religious beliefs Samurai and Shinobi believed in. The Shinobi also benefited from an ecosystem that is one of the most scientifically fascinating ecosystems in the mountains of Iga, so training in Shugendo a environmental adaptation training benefited them. The Mercenary label came from jobs they would take, where they would need to infiltrate, gain intelligence, and eventually rescue a political prisoner taken from a different feudal Lord. The Shinobi had two major sects Iga & Koga, and they had a delegation of 12 members chosen from the two clans, and it acted as a method of checks and balances to contain each other.
The Ninja idea or concept of modern times came after Oda Nobunaga, attacked Iga twice where he lost, and was embarrassed. His success came with the alliance with another feudal Lord, they invaded with 50,000 men burning the forests and villages, killing men, women & children. That caused the clans to disperse, since they where basically spy, warrior, farmers they had to take work to get resettled after losing their clan, and homes. This didn't eliminate Ninja who passed on the more traditional teachings, but the fallout created an environment where the more modern idea of what a ninja is could gain traction.
The uniting fact in martial arts, marksmanship, archery, or any other activity that is why every style of martial arts teaches a set of strung together moves is to create muscle memory. Each move is creates muscle memory, the method of stringing them together is to teach the individual moves to an individual based off their rank and skill level.
So you may learn Tar Kwon Do Chun A- each move repeated so muscle memory forms, but the entire "form" is taught because it's the optimal method to teach that level of moves into a person. Believe me, no matter how many styles and forms you learn, in real life situations, you will naturally react with the move you have trained in, your not gonna bust out a mortal combat combo spree and complete a full "form" on them using every move that you learned in it.
If you train and devote yourself to training you will react instinctually, but have the ability to respond until you have neutralized the threat, or you hope that is the result. The best fighter can make one misstep, and loose to a weaker untrained opponent, so don't ever stop learning, and training what previous lessons taught. If you think you can never be defeated, you will have negative consequences. Every time I spar or fight competitively, I think about how I could loose, winning isn't a guarantee. It keeps me from complacency, and from looking like a douche. Sorry