As France withdrew from a provisionally divided Vietnam in late 1954, the United States increasingly stepped in to support the South Vietnamese leaders due to the Domino theory, which theorized that if one nation would turn to communism, the surrounding nations were likely to fall like dominoes and become communist as well. The Soviet Union and North Vietnam became important allies together due to the fact that if South Vietnam was successfully taken over by North Vietnam, then communism in the far east would find its strategic position bolstered. In the eyes of the People's Republic of China, the growing Soviet-Vietnamese relationship was a disturbing development; they feared an encirclement by the less-than-hospitable Soviet sphere of influence.