Final answer:
The brutal actions of U.S. soldiers in Vietnam threatened the United States by causing international outrage, creating a deep divide within the country, and eroding public trust in government policies related to the war.
Step-by-step explanation:
The actions of U.S. soldiers in Vietnam during the war were perceived as a threat to the United States for multiple reasons. Firstly, the brutality and war crimes committed by troops, such as those described by John Kerry in 1971, led to international outrage and significantly damaged America's reputation on the world stage. Secondly, the military actions against civilians and the destructive tactics used, such as the use of napalm and the My Lai massacre, led to a divide within the United States and eroded public trust in the government's policies and handling of the war. Finally, the mistreatment of Vietnamese people and the broader implications on human rights highlighted a contradiction with American values, sparking internal conflict and widespread protests across the nation.