Most observers expected Chiang's government to eventually fall to the imminent invasion of Taiwan by the People's Liberation Army, and the United States was initially reluctant in offering full support for Chiang in their final stand. US President Harry S. Truman announced on 5 January 1950 that the United States would not engage in any dispute involving the Taiwan Strait, and that he would not intervene in the event of an attack by the PRC. Truman, seeking to exploit the possibility of a Titoist-style Sino-Soviet split, announced in his United States Policy toward Formosa that the US would obey the Cairo Declaration's designation of Taiwan as Chinese territory and would not assist the Nationalists. However, the Communist leadership was not aware of this change of policy, instead becoming increasingly hostile to the US. The situation quickly changed after the sudden onset of the Korean War in June 1950. This led to changing political climate in the US, and President Truman ordered the United States Seventh Fleet to sail to the Taiwan Strait as part of the containment policy against potential Communist advance.

In June 1949 the ROC declared a "closure" of all mainland China ports and its navy attempted to intercept all foreign ships. The closure was from a point north of the mouth of Min River in Fujian to the mouth of the Liao River in Liaoning. Since mainland China's railroad network was underdeveloped, north–south trade depended heavily on sea lanes. ROC naval activity also caused severe hardship for mainland China fishermen.

During the retreat of the Republic of China to Taiwan, KMT troops, who couldn't retreat to Taiwan, were left behind and allied with local bandits to fight a guerrilla war against the Communists. These KMT remnants were eliminated in the Campaign to Suppress Counterrevolutionaries and the Campaigns to Suppress Bandits. Winning China proper in 1950, also after Annexation of Tibet, the CCP controlled the entire mainland in late 1951 (excluding Kinmen and Matsu Islands).