In the early 1930s, the US adopted an isolationist foreign policy, and three acts were passed to keep the country out of war. However, with the spread of the Second World War, the US gradually moved from neutrality and tried to aid Britain after the fall of France.
The bombing of Pearl Harbor formally involved the US in the war. Then the US set its wartime objectives: to destroy the Axis powers and establish a new world order. Therefore, the US coordinated with British and formulated strategies to defeat Germany, and it also cooperated with the Soviet Union to lower its own loss while watching out for the Soviet Union's expansion. The guiding principles were to win the war and establish a postwar political structure.
In a word, the US adjusted its foreign policies to the changing situations for the sake of its own interests with great success.
20世纪30年代初，美国采取了孤立主义外交政策，通过了三项法案，使美国免于战争。然而，随着第二次世界大战的蔓延，美国逐渐从中立转向法国沦陷后试图援助英国。
珍珠港爆炸事件正式使美国卷入了战争。然后美国设定了战争目标：摧毁轴心国，建立新的世界秩序。因此，美国与英国联手，制定了打败德国的战略，同时也与苏联合作，降低自己的损失。指导原则是打赢战争，建立战后政治体制。
总之，美国为了自身利益，根据形势的变化调整了外交政策，取得了巨大的成功。
