Answer:
the geopolitical arena and the changes in the international system and a powerful public opinion against the war
Step-by-step explanation:
Before Vietnam, the United States managed to convince its internal factions to support the war effort in lands where little was to be done.
In a era dominated by ideological tendencies, the Americans saw themselves as promotors of liberty and human rights and free market economies, however in Vietnam cultural factors like religion, history of occupation. (by the French, the Japanese) turned the country very eager to defend against outside forces.
The problem that caused mobilization of ground troops to the Asian land caused a huge economic cost that would not turn into profitable. As the conflict extended the Americans began to tire and worn out , no longer being able to morally nor ideologically convince to stay in that scenario.
The public opinion keep putting pressure on a time where even the West wanted to turn the attention of the goverment to the people. The Americans own internal affairs demanded attention rather than pursuing a failed foreign policy outside.