What explains the presence of the united states in china at the start of the 20th century?

China–United States relations, more often known as U.S.–Chinese relations, Chinese–U.S. relations, or Sino-American relations, refer to international relations between the United States of America and the People's Republic of China (PRC). The partnership between China and the United States, in which each nation regards the other as a potential adversary as well as a strategic partner, has been described by world leaders and academics as the world's most important bilateral relationship of the century.