Japanese armor evolved across the medieval period in close response to changes in how war was fought. The ō-yoroi of the Heian and Kamakura periods was built around mounted archery: heavy, box-like, awkward on foot but stable in the saddle. As infantry combat became more important, the lighter dō-maru took over. And when European firearms arrived in the 16th century, Japanese armorers adapted again with the tōsei-gusoku — the "modern armor" featuring solid plate cuirasses designed to deflect or resist musket balls. Three armors, three different wars.
Ō-yoroi
The "great armor" of the Heian and Kamakura periods (roughly 900–1300) — the iconic heavy lamellar armor of the mounted samurai. The ō-yoroi was built around the demands of horseback archery: a roughly box-shaped torso section, a wide lacquered-leather skirt that fell over the saddle, separate sleeves, and a distinctive helmet (kabuto) with wide flaring neck protection. Tiny iron and leather scales (kozane) were laced together with silk cord into rigid panels, then lacquered for weather resistance.
The result was striking, distinctive, and very heavy — a full ō-yoroi could weigh over sixty pounds. It was poorly suited to fighting on foot (the silk lacing absorbed water and could become waterlogged in rain or river crossings) but it was extraordinarily protective for its intended use. Surviving ō-yoroi from the Kamakura period, often preserved in temples and shrines, are masterworks of medieval craft, with the silk lacing dyed in elaborate decorative patterns specific to particular families.
Dō-maru
The "body wrap" — a lighter, more practical armor that opened on the right side and wrapped around the torso. The dō-maru emerged in the late Heian period as armor for foot soldiers and lower-ranking warriors, where it was preferred over the ō-yoroi for its lighter weight and easier movement. By the Muromachi period (14th–16th century) it had been adopted by senior samurai as well, particularly those who increasingly fought on foot rather than from horseback.
Like the ō-yoroi, the dō-maru was built from laced lamellar scales, but it dispensed with the heavy box construction in favor of a flexible wrapping form that conformed to the body. It marked a transitional point in Japanese armor — halfway between the cavalry-oriented great armor and the modern infantry armor that would replace both during the Sengoku. By the time of the warring states, the dō-maru and its closely related cousin the haramaki were probably the most common armors on Japanese battlefields, worn by a large fraction of any army's foot troops.
Tōsei-gusoku
The "modern armor" of the Sengoku period — the iconic samurai armor most modern depictions actually show, with its solid plate cuirass, simpler construction, and elaborate kabuto. The tōsei-gusoku emerged in the mid-16th century in direct response to the arrival of European firearms in Japan in the 1540s. Where lamellar armor handled arrows reasonably well, it was vulnerable to musket balls; the response was to build cuirasses out of larger plates, sometimes solid one-piece constructions modeled directly on European cuirasses encountered through the Portuguese trade.
Tōsei-gusoku was lighter, simpler, faster to make, and more practical than its predecessors — all advantages in the constant warfare of the Sengoku, where armies of unprecedented size needed to be equipped quickly. It became the standard for the great daimyō armies of Oda Nobunaga, Toyotomi Hideyoshi, and Tokugawa Ieyasu. The famous elaborate kabuto helmets of the period — with their crests, horns, and dramatic shapes — were largely a tōsei-gusoku phenomenon, used for visibility and unit identification on increasingly chaotic battlefields.