Even China controls 68% mining power does not mean they will do a coordinated attack. What is the point they want to attack it if they have the majority of the shares? doesn't make sense to me.
Trade Disputes or Land Disputes are way more important to China, than BTC.
Only the right circumstances have to occur for them to exert their control of BTC for all of the world to see.
1 such possible incidence could be in territorial disputes which are more important to them than BTC.
https://en.wikipedia.org/wiki/Territorial_disputes_in_the_South_China_Sea
Hell, China still claims they Own Taiwan.
http://nationalinterest.org/blog/the-buzz/are-china-taiwan-heading-towards-conflict-17860
