Isolationism: The United States had a long tradition of isolationism, dating back to the country's founding. Many Americans believed that the United States should avoid entangling itself in European conflicts. This sentiment was reinforced by the fact that the United States was geographically distant from Europe and did not have any direct territorial or economic interests in the region.
Neutrality Acts: In the early 1930s, the United States passed a series of Neutrality Acts that further strengthened the country's commitment to neutrality. These acts prohibited the sale of arms to belligerent nations, and they also required that American citizens traveling on belligerent ships do so at their own risk.
Public Opinion: Public opinion in the United States was also strongly in favor of neutrality. Most Americans did not want to see the country get involved in a war that seemed far away and unrelated to their own interests. This sentiment was reflected in the 1940 presidential election, in which Franklin D. Roosevelt was reelected on a platform of keeping the United States out of war.
Economic Interests: The United States also had economic interests in staying neutral. The country was a major supplier of food and other goods to the belligerent nations, and it stood to lose a great deal of money if it entered the war.
Lack of Preparedness: The United States was also not well-prepared for war in 1939. The country's military was small and poorly equipped, and it would have taken months or even years for the United States to mobilize for war.
For all of these reasons, the United States wanted to stay neutral in World War 1. However, the country's neutrality was eventually tested by the sinking of the Lusitania and other German attacks on American ships. In April 1917, the United States declared war on Germany, marking the end of its neutrality.