The correct answer to this open question is the following.
America's transition from historical isolationism to a more interventionist foreign policy after WWII was a necessity of the United States federal government in order to protect its economic and political interests in the world.
Let's have in mind that at the beginning of World War I, United States President Woodrow Wilson announced that the US would maintain a foreign policy of neutrality. Things changed when the US intelligence intercepted the Zimmerman telegram and the sinking of the Lusitania ship by the German Navy.
Something similar happened at the beginning of World War II. President Franklin D. Roosevelt maintained a foreign policy of neutrality until the Japanese attacked the island of Pearl Harbor in December 1941.
During these wars, US businessmen and the federal government loaned millions of dollars to Western European countries and sent war supplies. Indeed, the US proposed the Marshall Plan to help reconstruction in Europe.
US interests increased with these issues and with the pass of time it got involved in issues in the Middle East, due to its particular interest in oil deposits in that region.