It has often been argued whether and how the USA would defend Taiwan against invasion because of strategic ambiguities in US policy regarding Taiwan. However, the specifics of America’s legal responsibilities regarding Taiwan may provide an unambiguous bottom line as far as US policy is concerned, because the USA as a democracy finds it almost impossible to infringe the US law. By confirming its legal burden, US policy regarding the defence of Taiwan can be reinterpreted in a more sophisticated way, with the extent of America’s responsibilities defined by its legal boundaries. And the US legal obligation to defend Taiwan by force was affirmed when the USA-ROC Mutual Defense Treaty was valid, but under the Taiwan Relations Act the US legal obligation only covers the provision of sufficient arms