What were the main factors that changed how samurai did battles from being Archers/Calvary to more fighting on foot with swords and spears? written by u/ParallelPain
If samurais were mostly horse archer, and those on foot are mainly using spears, then how come we get the “the katana” culture that is so popular today? written by u/MRBEASTLY321
The short answer: Peace.
There were changes in Japanese warfare, but rather than turning samurai from mounted archers into primarily swordsmen, they turned samurai from mounted archers into spearmen, foot archers, and arquebusiers. The key changes were, in a causal sequence as far as possible:
An expansion in the size of armies.
A reduction in the importance of cavalry. The number of cavalry increased far less than the total number of soldiers. Infantry became dominant.
The main type of sword used by samurai changed from the tachi to the katana. These had similar blades, and mainly differed in the suspension - a tachi was hung from the belt, edge-down, and a katana was worn at the waist through the belt, edge-up. Tachi suspension works best for cavalry, and katana suspension is good for use on foot (since the sword is away from the legs, and doesn't bounce around).
If anything, these changes would have reduced the use of the sword by samurai in battle. The tachi, as used by mounted archers, was a cavalry sabre, and would be used when in close combat, and left the samurai with a weapon once they had shot all of their arrows. Infantry using spears or other polearms would generally keep using those weapons unless they broke, in which case they would use their swords. Typically, infantry were less likely to use their swords than cavalry, so the dominance of infantry meant that swords were used less. (Once the gun was in common use, many cavalry switched from bow to lance as their main weapon, but this didn't change the importance of the sword as an additional cavalry weapon.)
Thus, skill with a sword was important on the battlefield. It was proportionally more important (compared to skill with other weapons) away from the battlefield - one was less likely to be carrying a spear/gun/bow if not on the battlefield or going there/coming from there. Also, on the battlefield, your comrades will help you, but away from the battlefield, you might be alone and dependent on your own skills. Many Japanese martial arts systems treated the sword as the foundational weapon, the one to learn first. From that first weapon, similar principles would apply to the use of other weapons. (Similarly, the longsword (i.e., hand-and-a-half sword) was the foundational weapon in the main Medieval German and Italian martial arts systems we know of. Elsewhere, staff or spear were common foundational weapons.)
Wearing his katana when outdoors, and wakizashi (and/or tanto) indoors, the samurai usually had a sword available. If there was a fight (not on the battlefield), the sword would often be the weapon of choice, simply because it was there. The sword would be used in duels, as well as in impromptu fights.
The shift from battlefield to civilian settings made the sword much more prominent and, by taking samurai away from their bows and spears, made them swordsmen.
Some movies do show battles where most of the samurai are using swords. Typically, both sides rush together as two mobs, get mixed up together, and pair off into separate one-on-one fights. This isn't realistic. It's also a common way for ancient and medieval European battles to be shown in movies (and still unrealistic in this context). Armies used formations in battle, not mobs.