Public Opinion: After the devastating experiences of World War I and the Great Depression, public opinion in the United States was strongly isolationist. Many Americans were determined to avoid entanglement in another European conflict.
Neutrality Acts: The United States government pursued a policy of neutrality in the early years of the war, passing the Neutrality Acts in the 1930s. These acts restricted the sale of arms to belligerent nations and prohibited American ships from transporting passengers or goods to warring nations.
Lack of Direct Threat: In 1939, the threat from Nazi Germany was not immediately perceived as directly affecting the United States. The focus of the war was primarily in Europe, and many Americans believed that the Atlantic Ocean provided sufficient protection.
Limited Military Preparedness: The United States military was not well-prepared for a major conflict in 1939. It lacked the troops, equipment, and resources necessary to intervene in a large-scale war in Europe.
Desire to Avoid War: Many political leaders and citizens in the United States were reluctant to enter another major war. The memory of the significant loss of life and destruction caused by World War I was still fresh, and there was a strong desire to avoid similar tragedies.
It was not until the Japanese attack on Pearl Harbor in December 1941 that the United States officially entered World War II.