Answer:
American Foreign Policy developed and expanded over time because the United States went from being a new and weak country, to being the sole superpower of the world.
Explanation:
The United States was at first a new country that had gained independence from the British Empire after a bloody war. During this period, the U.S. was mainly interested in keeping the British threat from reconquering the country, and consolidating internal stability.
As decades passed, and the country became larger and wealthier, it began to enact a more expansionary policy. This is represented by the Louisiana Purchase and the Mexican American War, which tripled the size of the U.S.
At the end of the XIX century, the U.S. defeated the Spanish Empire, and gained control of Puerto Rico and the Phillippines, setting a precendent.
Its intervention in both World Wars made the U.S. one of the two superpowers of the world, and the final break up of the Soviet Union in 1991, made it the sole superpower, allowing America to enact a more expansionary foreign policy.
