Short Answer 
The U.S. was primarily concerned about the spread of communism in the 1950s, especially in Vietnam, due to the ‘domino theory’ which suggested that if one nation fell to communism, others would follow. This concern was part of the Cold War context, leading the U.S. to implement proactive policies, including military support for South Vietnam, to curb communist influence in Southeast Asia.
Step 1: Understanding U.S. Concerns in the 1950s
The United States was primarily concerned about the spread of communism during the 1950s, particularly in Vietnam. This concern stemmed from the belief that if one country fell to communism, neighboring countries would follow, a concept known as the ‘domino theory.’ This anxiety prompted the U.S. to take actions aimed at preventing a wider communist influence in Southeast Asia.
Step 2: The Cold War Context
The Vietnam conflict was part of the broader Cold War struggle between the United States and the Soviet Union. Throughout this period, both nations sought to exert their control and influence globally, which often led to direct interventions in various countries. The U.S. aimed to maintain a balance of power by curbing the spread of communism, viewing it as a unified threat not only from Vietnam but from other communist nations as well.
Step 3: U.S. Policy Implementation
In response to these concerns, the United States adopted a proactive policy towards Vietnam by providing military and advisory support to the government of South Vietnam. The U.S. also carefully monitored the relationships between global powers, especially regarding China and the Soviet Union, while continuing to perceive communism as a singular threat. This perception heavily influenced their diplomatic and military strategies in the region.