Albeit this is a basic answer, World War Two was a major catalyst to an increased focus on internationalism. In addition, financial and political interests (most notably for political interests is the Cold War) are both legitimate reasons to why the United States of America has remained as a major part of international affairs.
A, they had no land of their own :)
I am guessing you are meaning after the Civil War, which if this is the case, it is known as Reconstruction.
