What war? If your referring to WW2 then because they were provoked from the japanese. IF they attacked the US, then they feared that later in the future they will be overrun when germany plus japan imperialism will be too strong for them.
When WWI began in Europe in 1914, many Americans wanted the United States to stay out of the conflict. So they won't get dragged into it plus it would cause drama between them and Europe and it would go on and on until the other team wins.
