The Japanese Ritsu order originated from the Chinese LŘ-tsung order (Korean = Yulchong) which was founded by Tao-hsuan (596-667). The doctrinal basis of the Ritsu order is the Dharmaguptaka (s)/Shibunritsu (j) version of the Theravāda Vināya/Discipline ("That Which Leads") rules which encompasses 250 Monk and 348 Nun vows.
While formulating the doctrine of the LŘ-tsung/Ritsu, Master Tao-hsuan visited Hsuan-tsang (600-664) in Chun-an where he was became impressd by Fa-hsiang doctrine. He thus integrated elements of the Mahāyana with the Theravāda Vināya and wrote commentaries to the "The Brahman Net Sūtra"/Fan-Wan (ch) and the "Treatise of the Accomplishment of the Yogi Masters"/ Wie-Jia-She-De-Lun (ch) which outlines 4 major- and 40 minor rules for Bodhisattvas. Tao-hsuan was of the opinion that the only way to ensure a rebirth in the human or heavenly realms was by practicing the Five Precepts and that the only path to achieve ultimate enlightenment was by following the Eight Precepts of the Sangha. (see below)
Accepting the Theravāda Vināya as authoratative, the Chinese Mahāyana widely accepted Master Tao-hsuan teachings and it became integrated into numerous Chinese Mahāyana orders.
The Ritsu was introduced to Japan, first from Paekche Korea by the japanese nun Zenshinni who returned to Japan in 590. The precepts of the Nanzan order were further transmitted during the reign of Temmo into the Tempyō period.
Later the monk Dōsen (j) Tao-hsŘan (596-667) re-introduced the Ritsu as well as Kegon teachings to Japan in 736, however was also forgotten.
At that time numerous orders had already been established in Nara but lacking the prescribed number (10) of fully ordained monks nescessary to carry out new ordinations, the continuation of the Japanese Sangha was in danger. Upon Ganjin's arrival, Emperor Shōmu (reign 724-749) approved the building of an ordination platform (Kaidan'in) at Tōdaiji and being able to fulfill the prescribed requirements for new ordinations Ganjin and the other Chinese monks of his entourage undertook the ordination of Japanese monks in 755. Other ordination platforms were established in 761 at Yakushiji in Shimotsuke (Eastern Province) and Kanzeonji in Tsukushi (Kyūshū).
Over time the Ritsu order waned in importance. Other (Mahāyana) Vināya traditions were transmitted from China and were made popular in Japan by Saichō who established a separate ordination platform on Mounte Hiei for these Mahāyana Endon precepts.
During the Kamakura period (1192-1333) the Ritsu became rejuvenated when the japanese monk Shunjō, having gone to China in 1189, returned from a twelve years stay in China. Having studied Tendai, Mikkyō, Zen and Ritsu he revived Sen'yūji Temple in Kyoto. Similarly the monk Eizon (1201-1290) transmitted the Nankyōritsu (Nara Vināya) to Saidai-ji in Nara. Tōshodaiji and also Kaidan'in continued to be active centers of the Ritsu.
In 1602 the Nankyōritsu (Nara Vināya) was introduced by Myōnin to Kōzanji in Kyoto, thus establishing a Shingon Ritsu order which like Eizon followed a Vināya based upon the the Indian Sarvāstivāda order.
The Ritsu waned in importance however over time became an important element in the Shingon whose most prominent advocates were Jiun and in the Edo period Jōgon and He Sonja Onkō.
In the 19th century the Ritsu order was, with the exception of Tōshōdaiji Temple, officially incorporated into the Shingon order by decree of the Meiji government.
I will refrain from: