The United States wanted to remain neutral in World War II because the federal government was still focusing on helping America recover from the Great Depression which began in the early 1930's. Along with this, the government did not want to get involved in an expensive overseas war.
However, this all changed when the American military base in Pearl Harbor Hawaii was attacked by the Japanese military. The bombing of Pearl Harbor on December 7, 1941 almost immediately resulted in the United States joining World War II. Only one day after this attack, President Franklin D. Roosevelt asked Congress for a declaration of war. The agreement to go to war passed almost unanimously in both parts of Congress, resulting in the US officially joining World War II on December 8, 1941.