At the close of George Washington's presidency, he cautioned future presidents and policy makers from getting pulled into foreign entanglements, wanting neutrality and free trade with all countries. He thought that rivalries would lead to unnecessary and costly wars, and that alliances would also force participation in wars that would not benefit the United States. Further, he thought that foreign allies would try to influence domestic politics in ways that may not align with the country's best interest. This put the country on an isolationist track for the first few decades of its history.
The Monroe Doctrine shifted the theme of U.S. foreign policy slightly away from the strict isolationism envisioned by Washington. This doctrine stated that while the United States would not interfere in Europe, they would oppose European attempts to colonize the Americas. This sentiment was likely influenced by the previous American struggle for independence, in combination with the 19th Century belief in Manifest Destiny. Manifest Destiny was not a codified policy, but rather the popular sentiment that Americans were meant to settle the rest of the continent, pushing the U.S. border from Sea to Shining Sea. The realization of this belief would have been hindered by European colonization of the region. Years later, the sentiment of the Monroe Doctrine was acted upon during the Spanish-American War, where the U.S. aided the Cuban independence movement. This war was the beginning of U.S. status as a world power.
By the end of the 19th Century, the United States was even more embroiled in world politics. In response to European countries forming spheres of influence in China, the U.S. began to advocate an "Open Door Policy", which stated that Chinese trade should be open to all countries equally. This policy position was sent to the main European powers, hoping that they would agree to the policy so that the U.S. would be able to benefit from trading in the Chinese markets. All countries agreed, with the exception of Russia. While this is in the spirit of Washington's wish for completely free trade, it is decidedly meddlesome.
The United States tried to be neutral when World War I broke out, and for several years they succeeded. However, by 1917 the U.S. had been dragged in by the sinking of the Lusitania, killing over one hundred Americans, and the Zimmerman Telegram, in which Germany offered it's support to a potential future Mexican invasion of the United States. At the war's end, Wilson was one of the four major players at the Paris Peace Conference, where the terms of the Treaty of Versailles were set. He proposed "Fourteen Points", whose main ideals were open agreements to peace, free trade, self-determination, and a League of Nations. While few of these points made it into the Treaty of Versailles, the League of Nations was established. However, the U.S. Senate did not approve the treaty, and therefore the United States was not a member. Here, there is a clear divide between a president wanting to have a more internationally-focused foreign policy and a Congress wanting to remain isolationist as much as possible.
Between World Wars One and Two, the U.S. maintained isolationist policies. As Nazi Germany gained power, Congress passed a set of Neutrality Acts (1935-1937) with the intent to keep the United States out of second European conflict.
When World War II broke out, President Roosevelt did not wage war, but he did aid the Allies by sending war-making materials. Due to the constraints of the Neutrality Acts and the public disapproval of giving weapons freely, Roosevelt created the Lend-Lease program. The weapons would be given in exchange for favors. For example, Britain agreed to aid the U.S. in creating a liberal economic order in the post-war period. After Pearl Harbor, on December 7, 1941, the United States joined the Grand Alliance.
After the end of WWII, U.S. foreign policy turned away from isolationism and towards interventionism. There was a strong fear of the expansion of communism through domino theory (if one country falls to communism, others will follow). In response, the U.S. practiced a strategy of containment, created several alliance groups, and coined the Truman Doctrine and Marshall Plan. The North Atlantic Treaty Organization (NATO), the South East Asian Treaty Organization (SEATO), and the Central Treaty Organization (CENTO) were largely created to build coalitions against communism. The Truman Doctrine pledge U.S. monetary support for Greece and Turkey, who were threatened by the spread of communism from the Soviet Union. Implicitly, this mean that the U.S. would support other nations that fought the spread of Soviet communism as well. The Marshall Plan was a set of economic policies intended to aid European recovery after the war, thereby preventing the spread of communism. At this point in history, the U.S. clearly had an idea of how they wanted the world to look and implemented policies that would further those goals.
Since the end of the Cold War, the overarching theme of U.S. foreign policy has been promoting democracy and more recently combating terrorism. As you can see by going through this timeline, the themes of foreign policy in the United States has shifted slowly but dramatically from its start as an isolationist state to an active world power.