Why do you think American Imperialism stirred debate? Which side of the issue (imperialist or anti-imperialist) had the best argument?

Why do you think American Imperialism stirred debate? Which side of the issue (imperialist or anti-imperialist) had the best argument? (for this latter, do not use presentist (looking at the past from our own perspective in the early 21st century) arguments – use the reasoning of the time when considering your response).