Thinking like a commie ...
... they need a crisis, real or manufactured
... false flag attacks on Chi-com civilian or commercial assets or vessels
... a natural disaster, like an earthquake, tsunami, or cyclone to which the Chi-coms can offer 'disaster relief'
... or, in the dark of night, some Taiwanese Government officials 'invite' them in
... a dispute of the borders of both nations' Exclusive Economic Zones
... there's always the rapid, overwhelming invasion while the West, and maybe Taiwan, are distracted with other things
... once the Chi-coms have troops on Taiwan, it will be too late for the West. They will have to decide if they want a hot war against a Global nuclear power.
... I suspect in the absence of fate, the Chi-com's will encroach upon Taiwan's sovereignty to provoke a Taiwanese miltary reponse that will 'justify' China's further military actions against Taiwan.
The Chi-com's will embed as many agents as it can in Taiwan so it can attack from the outside and the inside simultaneously.
Any direct conflict with China will need to be a coordinated, multi-lateral campaign of total war.