How is the monroe doctrine an example of diplomatic nationalism

The Monroe Doctrine, which was enacted on December 2, 1823, is an example of nineteenth-century nationalism. According to the Monroe Doctrine, any further interference by European countries with states or efforts to colonize land in the Americas would be considered an act of aggression. The doctrine went on to say that the US would not come close within existing European countries or meddle in their internal affairs and that the Western Hemisphere was no longer open for business.