20th Century Isolationism

Historically, the United States has often taken a stance of non-involvement. The Monroe Doctrine, which prohibited US involvement in Latin American affairs unless provoked, is a prime example of American non-involvement.
However, in the 20th century, non-involvement escalated into isolationism.
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The Monroe Doctrine
World War I had resulted in massive losses for the American military, as over 117,000 soldiers had been killed on foreign soil. After the war, the Congress rejected a proposal to join the League of Nations. This was done under the belief that the League would require the United States to intervene in future European conflict, resulting in the loss of even more American lives. This was ultimately proved to be an invalid excuse, as this was not a requirement for participation.
League of Nations

After the rejection of the League of Nations, the US passed a series of bills that disconnected itself from the rest of the world. This began with the Stimson Doctrine, which stated that the United States would not recognize territories that had been acquired through force. This policy was enforced on multiple occasions, including the Japanese conquering of Manchuria, the Italian invasion of Ethiopia, and the Soviet annexation of Latvia, Lithuania, and Estonia.

After the Stimson Doctrine, President Roosevelt was elected. Being of non-isolationist ideals, he proposed a bill that would allow the United States to convene with other countries to discuss a country that was being too aggressive. This proposal was immediately shot down by isolationists, who believed that the United States should not meddle in foreign matters, which could potentially result in an unwanted war.
President Roosevelt discusses neutrality

In the beginning of WWII, the Neutrality Acts were passed, which stated that the United States had no interest in the aiding either Britain/France or Germany. The neutrality of the United States was maintained until the sinking of an American ship by a Nazi U-boat. Unfortunately, all of the isolationist measures were not successful, as the United States went on to fight in World War II, during which 418,500 soldiers were killed.

In addition to political acts against world affairs, the United States also felt pressured to maintain a strictly "American" environment, especially after the Cold War. In the 1920s and 1950s, the "Red Scare" became prevalent. The first Scare, after the Russian Revolution of 1917, caused fear of anarchism and rebellion, and this fear was directed towards Eastern Europeans. The United States reacted to the Red Scare with the Sedition Act of 1918, which made it possible for the US government to forbid any inappropriate language about the government. This Act was received well by the public, who were highly fearful of anarchy.

Less politically correct means were also taken, as displayed with the Palmer Raids. The raids were executed by Attorney General Palmer and J. Edgar Hoover, who was promoted to head of a division of the Bureau of Investigation, and together, roughly 10,000 "dangerous radicals" were arrested, many of whom were immigrants. The raids have been long debated, as the true danger of many of those arrested can be considered questionable, and the First Amendment could potentially have been violated.

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A poster about the Red Scare
In the 1950s, Senator Joseph McCarthy re-established the Red Scare with his anti-Communist movement, which accused countless individuals of having Communist ties. This movement, known as "McCarthyism" is often described as witch-hunt, as it ruined countless reputations and even resulted in arrests.

During the Red Scares, the American people intended to maintain their country's values and not let them be corrupted by what they believed to be the "deplorable" system of Communism. This is an example of internal isolationism, as the United States government and people avoided any outside influences and maintained a strictly "American" values.

Isolationism and Sectionalism go hand in hand. With Isolationism, the United States itself becomes one section of the world and does not allow any outsiders to penetrate the American bubble. This is most often used as a defense against a war, as post-WWI, the US avoided any external communications that could result in further conflict, and after the Russian Revolution, a crusade against non-American values began. The two ideas are ultimately the same, however, isolationism takes place on a much larger scale.