Japanese Esoteric Buddhism
During the Tang Dynasty in China, when esoteric Buddhist practices reached their peak, Japan was actively importing Buddhism, its texts and teachings, by sending monks on risky missions across the sea to stay in China for two years or more. Depending on where the monk stayed and trained, they may have brought back esoteric Buddhist material and training back to Japan.
In 804, monk Saicho came back from China with teachings from the Tiantai sect, but was also trained in esoteric lineages. When he later founded the Japanese Tendai sect, esoteric practices were integrated with the Tendai teachings, but Tendai is not an exclusively esoteric sect. Subsequent disciples of Saicho also returned from China in later years with further esoteric training, which helped to flesh out the lineage in Japan.
On the same mission in 804, Emperor Kammu also sent monk Kūkai to the Tang Dynasty capital at Chang'an (present-day Xi'an). Kūkai absorbed the Vajrayana thinking from eminent Indian and Chinese Vajrayana teachers at the time, and synthesized a version of which he took back with him to Japan, where he founded the Shingon school of Buddhism, a school which continues to this day. Unlike Tendai, Shingon is a purely esoteric sect.