According to the U.S. Constitution, the foreign policy of the country is determined by the Executive (the President and the Department of State) and the Legislative (the Congress). The President, as the Commander in Chief, is entitled to send military forces abroad but only for a period of up to 90 days. For longer periods, such as full-blown wars, approval from the Congress is required.
From its founding after its War of Independence against Great Britain, following the Declaration of Independence in 1776, the United States followed isolationism, a foreign policy to which successive American governments tended to revert to until after the end of World War I in 1918, and beyond. Initially, the policy was intended to keep the new republic detached from costly and destructive conflicts with the more powerful European countries. Many of the young country's new citizens had emigrated from Europe to escape such conflicts.
The isolationist policy was enshrined by President James Monroe in 1823, in what became known as the Monroe Doctrine, which stated that any intervention of a European country in the Western hemisphere would be regarded as an act of hostility. At the same time, the U.S. Constitution proclaimed liberty and equality for all, and some Americans wanted to influence neighbouring nations according to these principles through a policy of internationalism.
For more than a century, these apparently conflicting principles of foreign policy were applied at U.S. convenience, with isolationism only toward European countries, while internationalism allowed America to launch invasions in Mexico, the Caribbean and Canada. This was explained in Theodore Roosevelt's corollory to the Monroe Doctrine, when in 1901 he said that in its relationships with neighbouring states the United States should "speak softly and carry a big stick," a policy he carried into his presidency.
When the United States did abandon isolationism, as it did in 1917 to join World War I, it did so only for a brief period. President Woodrow Wilson's attempt to position the United States as a world leader after the post-war Treaty of Versailles and through the League of Nations was rejected by Congress, and America refused to sign the treaty or join the League.
Isolationism prevailed until 1941, when the United States was forced into World War II following the Japanese attack on Pearl Harbor. Despite the efforts of President Franklin Roosevelt to assist the allied powers against Nazi Germany, Italy and Japan, there was opposition to any involvement in another "foreign war." In 1938, President Roosevelt had announced his four fundamental freedoms, which had to be enjoyed by all people: the freedom of speech and religion and freedom from want or fear. Roosevelt's definition of freedom and his strive to form a world where everybody has these freedoms would later result in the establishment of the United Nations.
The end of war saw the world effectively divided between two superpowers: the United States and the Soviet Union. With the UN headquarters established in New York, the U.S. this time did not disengage from global affairs. By 2010, the U.S. had become a leading member of around 80 international bodies, such as the International Monetary Fund and World Health Organization, many affiliated to the UN. After World War II, the United States emerged as the world's most powerful and influential nation, with the strongest economy; the Marshall Plan for funding the reconstruction of Europe helping to reinforce that status.
The U.S. needed to deal with the threats posed by the Atom Bomb and the Cold War, and in 1949 the U.S. led the foundation of NATO - the North Atlantic Treaty Organization, a military pact with western European powers as counter-balance to the Soviet influence over eastern Europe. Over the following 40 years, the U.S. was involved in numerous, often anti-Communist, conflicts, notably the Korean War (1950-1953), the Vietnam War (1955-1975) and the Cuban Missile Crisis (1962).
The Cold War ended in 1991 with the collapse of the Soviet Union. Internationalism, the idea of spreading democracy and liberty, coupled with the quest for economic progress, now drove U.S. policy, which saw its leading the 1991 Gulf War, supporting Kuwait against Iraqi invaders, and involvement through NATO in various Balkan conflicts in the 1990s.
After the terrorist attacks on the World Trade Center in New York on September 11, 2001, American foreign policy was focused on the "War on Terror," with U.S. participating in wars in Afghanistan and Iraq.