an interesting argument I have listened to as to why the US is not an empire is because traditionally empires have used, plundered their colonies to enrich the centre, the imperial heart... imagine Athens or Rome for example in ancient times or Britain, Russia, Spain etc. in modern times
but for all of America's cynical opportunism internationally, at least in modern times (ie. not the Louisiana Purchase or Mexican War), it has not actually - per the argument - enriched America in general but rather just classes within the US --> corporate America, the military industrial complex, the political elites etc.
now, if that is the case, it begs the question whether the US really is the greatest country in the world for the majority of their citizens? and if it isn't, if the wealth of the US is actually not meant for ordinary Americans through healthcare and social welfare and infrastructure and education and economic opportunity and standard of living etc., and there are plenty of social and political and racial and class divisions in the country, maybe things like mass shootings and crime and demonstrations etc. are more understandable
just consider the following picture