The United States followed the foreign policy of non-interventionism for most of the 19th century and well into the 20th century. After a quick involvement in World War 1, the United States retreated to stay out until 1941 when it actively joined World War 2. After the war, the worldwide spread of communism became a new danger and the United States became actively involved in European affairs through the Marshall Plan that offered aid to all democratic nations. The U.S. foreign policy became one of interventionism. Due to their foreign aid, many European countries became client states marked by their economic, political, or military dependence on the United States, During the Cold War, containment became an important part of U.S. foreign policy to prevent the spread of communism in Europe and Asia, as well as the policy of detente to minimize the risk of a nuclear war between the superpowers the United States and the Soviet Union. After the Cold War, the new world order was founded, which signaled a new cooperation between Russia and the United States. There were several wars in support of U.S. foreign policy interests including the Gulf War and the Bosnian War. After the terrorist attacks of 9/11 in 2001, the Preemptive War Doctrine led to wars in Afghanistan. The United States Foreign Service is part of the U.S. Department of States and assists U.S. citizens overseas and helps pursue the United States' foreign policy goals.