Since when US needs justification to wage wars and station its troops around the world?
There hatred is not a good thing for China, Japan is one of China's most important trading partners, they have their centuries old national feud tracing back to Ming dynasty 600 years ago and it had little to do with China, with or without China or US, They will still hate each other anyway.
not just Ming but in Tang DY too, what @mike2000isback said are bullsh!ts!
Since when Chinese army needs someone else's premission to fight in korean penesula?