The United States has maintained the long traditions of its foreign policy, from interventionism to isolationism. The United States intervened in World War I and II. The country had been greatly worried about its interventions in the wars of the largest scale. Former President Woodrow Wilson was forced to intervene in WWI after a long neutral stance, and President Franklin D. Roosevelt did so in WWII. He did not want to repeat Wilson's "mistakes." He successfully established postwar programs with the whole-hearted support of Congress. Traditionally, isolationists had, however, opposed the two interventions. The U.S. Senate of the Wilson administration rejected the Versailles Treaty in fear of being entangled in another conflict in Europe. In the first years of Franklin D. Roosevelt's presidency, there was agreement that Americans should go no further than minimal intervention and cooperation. From the late 1930s until American entry into WWII in 1941, there was again conflict within the decision-making elites over intervention. Isolationists were very intransigent about the League of Nations and later objected to Roosevelt's pro-Allied stance, which would eventually bring the United States into the war against Germany. Roosevelt was, nevertheless, forced to go to war against Nazi Germany and immediately in the wake of preemptive attacks on the Pearl Harbor by Japan. Roosevelt's diplomats argued that a Nazified Western Europe would pose a direct threat to the security of both the United States and Europe. They believed that the United States was dependent on democratic values in Europe. The United States made great efforts to rebuild European security through the Marshall Plan and NATO. Americans suffered from these hard efforts. It cost the United States a great deal, both economically and militarily. Americans sacrificed many lives in the Korean War, contributing to rebuilding the Korean economy and military strength, although the Eisenhower administration was satisfied with establishing the status quo along the 38th Parallel. Moreover, the United States made greater efforts to protect democracy in the Western world during the long Cold War. The Cold War reflects harsh competition between the two systems of democracy and communism. Today, they are suffering too much in the long mired tugs of war in the Islamic world, for which the United States has squandered billions of dollars over the past three decades or more. They are now faced with IS terror attacks within and outside the U.S. territory. Today, President-elect Donald Trump tries to turn away from longstanding U.S. practices in international conflicts. He said many times during his presidential campaign that he would make a great United States if he would be elected as president. This means that the United States should no longer play the role of world police and that Americans have been sick and tired of this role, which has cost too much economically. They believe that their sufferings resulted mainly from the globalization based on liberal trade and on chasing away protectionism. Judged from this point of view, Trump is, for better or worse, expected to rearrange free trade agreements such as NAFTA, the Korea-US FTA, the TPP, etc. These institutions are biased or unilateral, reducing the traditional U.S. industrial and financial capacities and consequently bringing about high levels of unemployment. At last, this has leveled down the U.S. prestige and morale of Americans. He now seems to think that removing protectionism alone would bring Americans better lives morally as well as economically, and that blocking immigrants would secure security in the American society. It is believed in his mind that such actions will be the only sure way to reinvigorate the once frustrated morale of the Americans. The other factors for their sufferings are in the role of the world police-maintaining peace and security. The U.S. administrations traditionally have played this role as superpower. U.S. leaders have thought that the U.S. role of peace and security provider alone could secure the world s safety and sustainably maintain democracy in the world. This role would, they believe, break down the continuing excessive expansion of China and Russia. Trump believes that this role will no longer work and he should focus on rebuilding the crumbling American society, consequently making a "Great U.S."again. His isolationism augurs ill, especially with respect to illegal immigrants. It aims to deport all of them, and it blames Mexican immigrants who are treated as black workers, drug-dealers and rapists, etc. He wants to construct a substantial wall on the Mexican-U.S. border at Mexican expense. Trump hates Islamic immigrants. Upon his presidential inauguration, he is expected to apply a temporary ban of foreign Muslims suspected of being terrorists originating from countries with a history of terrorism against the United States. Judging from his isolationist stance, he seems to ignore the nuclear and missile development issue of North Korea. The country is viewed as a rogue state that does not respect human rights and international morality. The United States has 5,000 nuclear warheads and 1,000 intercontinental ballistic missiles, while North Korea is assumed to have 10 to 20 nuclear warheads. He believes that North Korean nuclear development is its own domestic problem and not as a global one. His ideas about NPT appear to be a narcissistic realism that South Korea and Japan can make their nuclear weapons if and when they want. He seems to ignore the nuclear domino effects in Northeast Asia. Trump might have talks with Kim Jong-un if he asks first. He may, however, continue strategic ignorance on the issue. Such points of view reflect irresponsible realism aimed at especially American interests. Trump thinks that such a concentration of American energy on domestic development practically needs new isolationism. Viewed from this point of view, he is supposed to go back to minimal cooperation - an isolated position in the international community - which would be more concerned about American lives than ever, in disregard for the word police role. It should be, however, noted for Trump that the G20 conference held in September 2016, in Hangzhou, China agreed to the free trade order that aimed to reject the rising protectionism. The conference meant that the protectionism would restrain multi-free trade and conversely increase conflict. To be frank with Trump, the rebuilding of a "Great United States" will not be easily in the hands of Americans if global networking continues to be ignored. The protectionism as irresponsible realism is the principal source of hurdles to sustainable economic growth and peace. The present world has already entered the global networking in which the world economy operates and world peace is ensured. Viewed from this standpoint, new isolationism in the 21st century is rather a bad omen.