Why China and Japan can't get along? Were you sleeping in the stone age and just woke up? I suppose many foreigners are totally clueless about the ancient relations between China,Japan and Korea to this very day. Many don't know that the modern Japanese (many of them) have Chinese blood
since Chinese migrated to Japan and mixed with their locals since the Qin Dynasty. Chinese culture has influenced the Japanese immensely (food, writing system, clothing, architecture, garden, tea, buddhism, eating with chopsticks, weiqi, color carp breeding, art etc..) Okay the hentai,adult entertainment industry and anime have nothing to do with us
(that's typical Japanese culture, jokes aside). The cultural influence on Korea is the same, many things originated from China including the writing style until the Koreans adopted their own.
We had good relations with Japan, but Japan had tried to invade Korea few times and with the help of China we kicked the Japanese back to their island. Plus Japanese started to get arrogant and saw their empire as an equal to the Chinese empire. So yeah there were ups and downs between Chinese/Koreans with the Japanese. You have to understand Japan geographically speaking has no resources, constantly plagued by earthquakes and tsunamis. So they wanted to get on the Mainland Korea. It was during the Meiji era the Japanese went to the West and copied its tech and started to industrialize. With their militarization the Japanese tried to conquer East, South East and South Asia during WW2. The Japanese committed crimes far worse than the Nazis in China and that's the main reason why many Chinese and Koreans included dislike the Japanese. Many books and youtube videos are available of what the Japanese did so you can do your own research.