At the start of World War I, America took a neutral stance, later entered the war in 1917 with several reasons.
In January 1918, President Wilson proposed his Fourteen Points as the basis for peace negociation, but the U.S. Congress refused to ratify the treaty.
In 1929—1933， America had an economic depression. Roosevelt was elected President and introduced the "New Deal", helping the country get out of the depression.
In the early days of World War II, the U.S. government adopted a site-on-the-fence policy, and the American capitalists wanted to continue their profitable trade with the warring countries, including the aggressors.
In 1941, America declared war against Japan. In 1944, America dropped two atomic bombs on Hiroshima and Nagasaki. On August 14, Japan surrendered and World War II ended.
In April 1945, a conference was called in San Francisco to organize the United Nations. During World War II, the development of the America economy reached a higher stage.
The Cold War：In the spring of 1947, Pesident Truman declared the "Truman Doctrine", marking the begining of the Cold War. In 1949, the U.S. formed the North Atlantic Treaty Organization.
The Korean War(1950-1953), The Vietnam War(1954-1975), the war greatly weakened America and sharpened the country's internal contradictions.
Nixon：reestablishing U.S. relations with China and negotiating the first Stratebic Arms Limitation Treaty with the Soviet Union.(Watergate Scandal)
Reagan：called for tight money policy, leding to an increasing gap between rich and poor, and socoal welfare programs had been drastically curtailed. At the end of his administration, the nation was enjoying its longest recorded period of peacetime prosperity.
Bush：joined the war between Kuwait and Iraqi, leding to faltering economy.
Clinton：signed the North American Free Trade Agreement.
