I'm sorry, but this question goes way too far into modern political commentary for us to allow it here. If you're interested in a more modern discussion of why the US continues to "be the world police," please feel free to check out /r/AskSocialScience. If you're interested in how the United States emerged as the pre-eminent superpower, please resubmit your question, while keeping in mind that discussing the last 20 years in this subreddit is prohibited.

Thanks much! :)