With Russia, China and Japan building up their militaries. (Japan had to change their constitution in order to do so.)
Now lets not forget about India who is also keeping pace with China.
I could see an alliance forming with Japan and India with S. Korea taking advantage of this by bringing more pressure on N. Korea. Add all of the smaller countries on the Japan-India side that have felt threatened by China and we are look at a huge war.
If India and Japan attack China ,don’t be shocked if Russia attacks China as well. Having a weak China at the border would allow Russia to put more pressure on Europe. Russia would not even have to get into a shooting war, just mass troops at the border.
Now the why, its pretty simple , this war will be fought over passed insults and crimes as well as resources.
( I will finish this latter posting only to make sure I do )