Although China was the center of civilization in East Asia and a major influence on Japan, much of that influence came to Japan through Korea rather than directly from China. If Chinese influence was responsible for major changes in Japan – the import of Buddhism, writing, Confucianism, and technology – then Korea was the active conduit through which all of these came. The extent of Korean influence on Japan is not well understood; the two countries have fought with each other and mistreated each other as often as they have helped the other. Thus, the Japanese typically downplay their connections with Korea and Koreans emphasize how much they have suffered from the Japanese, particularly during the period when Korea was annexed by Japan, 1909-1945.
Korean influence on Japan goes back to the migration of people to Japan. It is very likely that the major imput into the Japanese racial stock is from or through Korea; recent readings of early history show that connections between the two countries were very close from at least the 4th century on. As late as the early 800s, a survey of important families shows that out of 1,182 noble families, 247 were from Korean kingdoms (176 were from China).
Japanese rulers frequently sent military aid to Korea, perhaps to help relations with kingdoms there. In the middle of the 8th century, as many as 41,000 troops were gathered to send to Korea but never departed. The Koreans also undertook large scale military action in 1274 and 1281 when they provided help to the Mongols who, having conquered China, tried to invade Japan. The invasions failed, thanks in part to typhoons and bad weather which played havoc with the invasion fleets. Later, in 1592 and 1597, Toyotomi Hideyoshi invaded Korea with initial success but final failure.
Aside from military action, Korea and Japan were constantly in touch through trade, piracy (the two were often hard to distinguish), diplomacy, and peaceful attempts to transmit culture and technology across the narrow strait separating the two. Piracy often complicated diplomacy, but relations based on the Chinese tributary system were maintained most of the time. The invasions often pushed Japanese into Korea and pulled Koreans into Japan; Hideyoshi’s invasions in the 16th century led to many Korean artisans being forced into Japan, to the great benefit of the Japanese pottery and ceramics industry. Trade relations continued throughout the Edo period with 20-50 Japanese ships a year engaged in authorized trade to Korea: more probably did illegally.
Much of the knowledge which the Koreans passed to Japan came originally from China. However, had the Japanese relied only on Chinese sources for outside influence and stimulus, the extent of Chinese influence would have been far less than it is; the sea routes to China were much longer and more dangerous than those to Korea and for centuries at a time direct contact with China was impossible due to political weakness in Japan. Contact with Korea was never broken for long, so Chinese influence, filtered through and moderated by Korea, continued.