In the 1800's Amerca mostly focused on developing itself as a country and did not engage much in foreign affairs.
After World War I, American foreign policy seemed to be one of isolation.
By the early 1900's President Franklin Delano Roosevelt sought to make a change to this stance of isolation due to his belief that doing so would aid the American economy in recovery after the Great Depression.
Roosevelt took it upon himself to meet with heads of the Soviet Union and establish a way to ensure friendly foreign relations with the Soviets and allow businessmen from America to buy and sell within their market. He also withdrew troops from Latin America and handed back control to their governments.
President Roosevelt established the Reciprocal Tariff Act of 1934 which promoted trade between foreign countries and the United States (a tariff is a tax on buying and/or selling of goods). The Tariff Act of 1934 allowed for a reduction of tariffs on American goods sold in foreign countries with a reciprocal reduction on tariffs of those same countries' goods sold in America.
After World War II, the US emerged as a powerhouse country. It took a leadership role in helping to found the United Nations, invested money, and built multiple alliances. Their main focus was containing communism. The outcome of WWII led to the collapse of Nazi Germany, the fall of Japanese and Italian empires, Allied forces victory, and the US and the Soviet Union emerging as rival powers.
The fall of the Soviet empire led to the US emerging as a lone power.
