The World War I had totally devastated the United States economy so being isolated was the only way to avoid foreign entanglements.
The United States proclaimed its neutrality and isolationist policies while Europe was being split up and fighting.
They are closely related. Ethnocentrism is when you are so wrapped up in your own culture or society that you are not really aware that some people behave differently. For example, some Americans might speak to a person from another country and ask "Who is your president?" not being aware that not all countries have presidents, or indeed that American law is not in force outside of the borders of the United States. Isolationism is a deliberate foreign policy to not involve one's country in the issues of the wider world. China pursued an isolationist policy for centuries. Having an isolationist or imperialist foreign policy can lead to ethnocentrism.
The question is unanswerable if the country whose foreign policy is being asked about is not specified. There are around 200 countries in the world.
President Hoover went with the flow of the American people. After the first world war, the American people were hesitant when dealing with European affairs; thus America entered an "isolationist" period. Hoover did very little with foreign relations until the very end of his term. He realized that being on good terms with Latin America was important to help with guarding the wester hemisphere. FDR would then take this idea to the next level with his "good neighbor policy".
Because they were natural.
Being Social - 2013 America's Role on Foreign Policy Now 1-7 was released on: USA: 21 June 2013
Nazi leader Hermann Goring had approached the Polish foreign minister in 1934 concerning a joint anti-Bolshevik foreign policy. Foreign minister Beck of Poland turned that idea down. It was clear that with Stalin being Poland's eastern neighbor, such a policy would be dangerous for Poland.
The Truman administration's foreign policy was to provide support to Turkey and Greece with military aid as well as economic aid. They did this to prevent Turkey and Greece from being taken over by the Soviets.
The strongest voice in shaping American foreign policy is typically that of the president. Unlike a senator or congressman who may want an aggressive foreign policy due to warships or fighter planes being located in his/her district, the president supposed to take a more nuanced view as to what is best for the country. Equally important, the president has the ability to deploy troops for combat without calling for a declaration of war, which is a huge voice in foreign policy.
The fact that Vietnam was being taken over by a dictator was one of the foreign policy issues that motivated American intervention in Vietnam. At first, the Americans were just there to teach the people of South Vietnam to defend themselves through the establishment of an army. There was also the Containment Policy, which was the United States' foreign policy to prevent more countries from "acquiring the disease of Communism".
Department of State
America had always been "isolationist" insofar as its circumstances allowed. Nobody wanted to be involved in European affairs more than could be avoided. In 1917, isolation became temporarily impossible when the Germans started sinking American ships, and sought a deal with Mexico against the US, forcing President Wilson to abandon neutrality. With that problem apparently taken care of, US policy went back to normal after the war. Much the same was true for Britain, which had pursued a policy of "splendid isolation" until the growth of the German fleet caused her to look for allies. With the German Navy gone, she too played only a very limited role in European affairs for much of the interwar preriod. Basically, the whole English speaking world was as "isolationist" as it could get away with being.