There were a number of them. You can start with the physical threat to China's borders and very existence, when a large American army approached through North Korea. This would get China into the war on North Korea's side, and last three years with over 1 million Chinese casualties.
Next was America's refusal to recognize communist China as the legitimate government, and preventing them from taking China's seat on the brand new United Nations.
The US also armed Taiwan with modern weapons and pledged to defend the island from communist takeover when we sent the navy in to Quemoy and Matsu in the 1950s.
China also did not become a nuclear power until the early 1960s, so it faced a disadvantage in competition with the United States and had to rely on nuclear protection from the Soviet Union, which it did not always see eye to eye with.
Lastly, China struggled under the weight of its huge population, and did not become anywhere near self-sufficient for food until the 1980s, and it could not trade with the largest agricultural exporter, the United States, until that time or later.