What happened
King Seong of Baekje, a kingdom on the Korean peninsula, sent a Buddha image and sutras to Emperor Kinmei. The Nihon Shoki dates the event to 552; other early sources give 538. Both dates are carried by tradition.
Acceptance was contested at court between the pro-Buddhist Soga clan and the opposed Mononobe clan. The Soga prevailed in the 580s.
Background
Buddhism did not travel alone. It reached Japan as part of a whole continental package — writing, architecture and statecraft — transmitted through the Korean peninsula.
Consequences
Early temples followed: Asuka-dera (596), Shitenno-ji (traditionally 593, founded by Prince Shotoku) and Horyu-ji (607). Over centuries Buddhism fused with native kami worship (shinbutsu shugo), laying the foundation of Japanese temple culture, art and thought for the next millennium and a half.