China's military power
I doubt they'll have the ability to pull off a successful invasion of Taiwan for some time. A massive amphibious operation like that is beyond their current means. Not the same as a land invasion of Korea for example. They could shoot missiles at Taiwan but I doubt that would acheive the aims they desire.
Seems to me the major stumbling block for Taiwan and China is the commies still being in power. Once mainland China becomes a full fledged democracy reunification will become more reasonable to the Taiwanese. Just my opinion.