Why was Japan never invaded by China or Korea?

Japan had nothing China needed or wanted. Japan is almost hilariously resource-poor. Its (somewhat unjustly) renowned katanas were forged the way they were precisely because the only iron available was of extremely low quality and that was basically the only way they could get a serviceable blade. read more

Had China had a formidable and unified state like they did in Sui dynasty (when they briefly and unsuccessfully invaded Korea) I would argue they still wouldn't have attempted an invasion of Japan because the thought of making this extremely foreign and “un-Chinese” place part of China was unheard of at the time. read more

And talking about Korea, it was a lot smaller and weaker than Japan historically. Thus, Korea was the one that was invaded, not the one invading. Japan did multiple invasions on Korea. Since the end of the Cold War, Beijing has used nationalism to shore up popular support. As a result, younger Chinese are often more anti-Japanese than older ones are. read more

Japan is an island nation just like the UK is and just like Japan, the UK has been invaded a lot less than other European nations. The fact is that an island nation is separated by the sea so any invasion force first has to secure naval supremacy (nowadays air supremacy too) in order to invade the island. read more