First, an often a resource, Wikipedia:
The word imperialism originated from the Latin word imperium, which means supreme power. It first became common with its current sense in Great Britain, during the 1870's and was used with a negative connotation.
When did it begin, “The Age of Imperialism”?
A time period beginning around 1760, saw (generally European) industrializing nations, engaging in the process of colonizing, influencing, and annexing other parts of the world in order to gain political power. Although imperialist practices have existed for thousands of years, the term "Age of Imperialism" generally refers
to the activities of European powers from the early 18th century through to the middle of the 20th century.
What is American Imperialism?
The economic, military and cultural philosophy which states that the United States either directly or indirectly affects and controls other countries or their policies.Can American Imperialism end, be curtailed?
In The Decline and Fall of the American Empire, Alfred W. McCoy write:
The demise of the United States as the global superpower could come far more quickly than anyone imagines. If Washington is dreaming of 2040 or 2050 as the end of the American Century, a more realistic assessment of domestic and global trends suggests that in 2025, just 15 years from now, it could all be over except for the shouting.Decline and Fall of the American Empire