pre-world War I
The United States used Isolationism throughout much of the 19th century and continued to exercise this policy in the years leading up to World War I. The policy was adopted to prevent United States involvement in conflict on European soil, as described in President James Monroe's "Monroe Doctrine." The goal of the doctrine was to prevent European nations from colonizing American land, thus leading to the development of U.S. isolationism. Any attempt by foreign nations to colonize land would be viewed as an act of aggression, and the U.S. would react with violence. The threats created by the doctrine would have proved ineffective without the help of the naval power of Great Britain enforcing the rules. The doctrine was established in order to prevent European countries from seizing territory that could prove beneficial both economically and geographically to the United States. It was used in full effect with the outbreak of the Spanish-American war in 1898, in which Cuba attempted to gain independence from Spain. Because the Monroe Doctrine established a goal of eliminating and preventing European influence in the Americas, the U.S. supported Cuban independence. When President Theodore Roosevelt was inaugurated in 1901, one of his major declarations was to update the Monroe Doctrine; he did this by adding the Roosevelt Corollary. This further ensured that European nations would not get involved in America's sphere of influence by declaring that the U.S. would help stabilize the economies of Central and South American countries. This would allow those countries to pay off international debts, not instigating European countries to use force if those countries could not pay off creditors. Cuba's success, backed by American firepower, and the Roosevelt Corollary led to the emergence of the U.S's involvement in foreign affairs. As tensions in Europe began to rise, it became more and more difficult for the United States to remain an isolationist country.
In the wars of the European powers, in matters relating to themselves, we have never taken part, nor does it comport with our policy, so to do."
World War I
As the war began, the United States' immediate reaction was to remain neutral by trading with both sides. This was the first step in the abandonment of the isolationist policy because remaining neutral in such a tense conflict was difficult. The U.S. began to give more to the Allied powers, prompting a growth towards them. Isolationism was officially renounced when the U.S. entered the war against Germany following the sinking of the commercial ship Lusitania, which killed 128 American civilians. Because this was a restricted submarine attack, the United States threatened Germany militarily, and Germany promised to stop performing restricted U-Boat attacks. Before declaring war, President Woodrow Wilson passed the National Defense Act in 1916, authorizing a large expansion of the military. This proved that the U.S. was ready to be involved in the war and attack Germany. After Germany broke its promise by performing submarine attacks on British and American merchant ships, the U.S. was forced into the war, completing its abandonment of isolationism. As the U.S. became more involved in World War II, President Wilson changed his motives for the war. A new policy was adopted to preserve democracy in the United States and to prevent Germany from expanding its territory. Through support from the United States and military success, Germany was defeated and the Allied Powers prevailed. Although the U.S. was in the war for a short amount of time, over 300,000 lives were lost, leading to many American citizens shifting from support to hostility towards war.
Prior to the official end of World War I on June 18, 1919, President Woodrow Wilson spoke before Congress regarding his vision of the world after World War I. This is known as Wilson's Fourteen Points, and one of the major components of this speech was the establishment of a League of Nations to prevent another major war from occurring. This was formed, but the U.S. did not join due to the majority of American citizens wanting to revert back to an isolationist policy. Due to heavy casualties and economic losses during World War I, the view of Americans changed from wanting to be involved in the war to cutting off ties with foreign countries. This led to the United States retracting back into the confines of its borders and re-enacting isolationism. Isolationism was taken to an extensive level, as shown by the tariffs imposed on imported foreign goods. Following the failure of the Prohibition and the Great Depression, the United States struggled to find an identity, especially due to differing public opinion on foreign policy. Isolationism continued into the 1930's, and its popularity was bolstered by the U.S.'s need to solve domestic problems, particularly a struggling economy. As tensions began to rise in Europe due to Germany's aggressive imperialist motives, Congress passed several acts to ensure that the U.S. would remain isolated. For example, one act prohibited the sale of munitions to foreign countries involved in conflict, while another declared that American citizens could not extend loans to countries entangled in conflict. Although Congress and the majority of American citizens supported firm isolationism, the growing threat of Japan and Germany swayed some to interventionism. The outbreak of war in 1939 showed that many Americans supported offering the sale of munitions to the Allied Powers. However, when American pride and safety was assaulted with the bombing of Pearl Harbor on December 7, 1941, Americans quickly favored abandoning isolationism to fight the Japanese.
World war II
Following the United States' entrance into World War II, the shift to containment and the abandonment of isolationism had begun. The passing of the Lend-Lease Act declared that the U.S. supply munitions to Allied Powers in an effort to help defeat the Axis Powers. As the U.S. continued to battle Japan in the Pacific and offer economic and military aid to Allies on the European front, many Americans began to support global involvement. While the U.S.'s major motive in joining the war was to seek revenge on Japan, it was evident that the territorial expansion of Germany was dangerous to the whole world; U.S. involvement in the war was inevitable. Along with a shift to a more globally involved foreign policy, the U.S. also began to use its military more extensively. Prior to the war, several jobs were created and the economy was boosted through manufacturing buildings which built only military weapons. Following the nuclear attacks on Japan, the United States had emerged as one of the world's top superpowers, eventually leading to containment and the current policy of interventionism. The Allied victory in WWII led to the emergence of the Soviet Union and the United States as global forces and began a conflict that would lead to the emergence of America's most famous foreign policy.