Revenge for WW2 but I still wouldn’t pick a fight with Japan!
And China shouldn't. Japan beat China in their 1895 war, and would have occupied all of China except for intervention by western powers that limited Japan to occupying Korea. Japan beat China in WWII except for intervention by western powers (leading to utter defeat of Japan by the U.S.A.).
This time around, there won't be western powers coming to the rescue of China. These conflicts between China and Japan have gone on for millennia. Japan remembers past injustices of Japanese women being kidnapped and tied to the bows of Chinese ships. The revenge goes both ways, as it does for conflicts between other hostile peoples (Turks/Armenians etc.)