The correct answer is D) To protect against an aggressive Soviet Union in Europe.
After World War II, the United States developed a policy of containment. This foreign policy was focused on stopping the spread of communism all over the world. To do this, the United States helped in creating the North Atlantic Treaty Organization (NATO). This organization would be made up of several different American allies (like France, Britain, Denmark, etc.) who would support each other militarily if the Soviet Union were to invade any of their countries.
