Since losing the mainland to Communist conquest in 1949 (more accurately, since the North Korean invasion of the South in June 1950), Taiwan has become a continuous foreign policy protectorate of the United States. Had it not been for American security protection, Taiwan would long since have come under Beijing's rule. Several causative agents, separately, in combination or sequentially, kept Taiwan out of mainland Chinese hands. These included, initially, the American Seventh Fleet, then generalized American military might in concert with the American-Taiwan Defence Treaty of 1954, thence the three American- Chinese communiques forming the basis of post-1971 relations between the two countries, concomitantly the American Congress's Taiwan Relations Act of 1979 and the accompanying (and subsequent) legislative history, and, throughout, China's inability to overcome, with a high probability of success, active Taiwan military resistance and probable American military support. While the economic and, more recently, political transformation of Taiwan materially strengthened that entity such that its defensibility against attack rose greatly, to say nothing of its overall attractiveness, from the onset of the People's Republic of China it was the American connection that was the sine qua non of Taiwan's quasi-independent existence.