(Not everyone likes tea….)
When people think of Japanese or Korean food and drink, tea often comes to mind, but tea as a drink has not always existed in Japan and Korea.
Tea was brought to Japan in the 8th or 9th century, as a medicine. Tea was used in Chinese medicine, and the Japanese mission to China (kentōshi 遣唐使) brought it back as the latest trend from China. However, its use was very limited and not well-known.
However, green tea was popularized by the founder of Rinzai Zen, Eisai:
Like many monks of the Nara, Heian and Kamakura periods, he traveled to China during his career to learn to study Chinese Buddhism, and bring back sutras, texts and training in monastic discipline. Although Eisai was ordained in the Tendai sect in Japan, in China, he was trained in meditation under the Linji lineage (Linji = Rinzai) and brought it to Japan where it became rinzai-shū (臨済宗) or Rinzai Zen.
Eisai wrote a number of health-related texts on green tea. The first one is the kissa yōjōki (喫茶養生記) which explained green tea as the ultimate medicine for maintaining health. The shogun at the time, Minamoto no Sanetomo, was so impressed that tea became a nation-wide drink.
Later, under the shogun, Ashikaga no Yoshimasa, tea culture flourished to its highest-form in Japan.
Korea, because it was next to China, had the benefit of easier access to Chinese culture and food, unlike Japan, so tea appears much sooner and in greater variety than in Japan.
According this site, tea appears in Korea as early as the 7th century, at least in the Kingdom of Silla. It was often used again in a Buddhist context similar to Japan, and official ceremonies.
However, over time, tea drinking recovered during the Joseon Dynasty and became widespread, and a cultural art form. Korean culture even has a tea ceremony, similar to the one in Japan but it described as less former and more relaxed. I found a cool video demonstrating the Korean tea ceremony, with nice traditional Korean music in the background:
Amusingly, it was actively opposed in another tea-drinking country: England. A book written in 1899 called Social Life of Scotland in the Eighteenth Century by Henry Graham said:
The fashion of tea-drinking, becoming common about 1720, had to make its way against vehement opposition. The patriotic condemned tea as a foreign drink hurtful to national industry ; the old-fashioned protested against it as a new-fangled folly ; the robust scorned it as an effeminate practice ; magistrates, ministers, and energetic laymen put it in the same malignant category as smuggled spirits, anathematised its use by the poor, among whom (they warned them) it would assuredly produce “corruption of morals and debility of constitution.”