When it's in their best interest to do so. The greatest challenge to China's leadership is within its own borders.
China is already stoking the fires of nationalism to deflect internal dissent, as Germany did. Taiwan is a total no compromise issue.
You have far, far too much confidence in the willingness of China to abide by the rules of others. Why should they have any more regard for American geopolitical rules than they do for copyrights and patents ? If power is there to be grabbed, China will grab it.