How did the monroe doctrine assert american nationalism

The Monroe doctrine asserted American nationalism by proclaiming that any further attempts by European powers to colonize or interfere in the business of places in the Americas would be seen as an act of aggression by the US. The Monroe Doctrine states that any country has the right to defend itself from unwanted outside forces. The Monroe doctrine also promised to protect any other country within the Americas, should they be attacked or an attempt was made by any European country to colonize it. This was a very generous offer to all third world countries within the Americas. 

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 The Monroe Doctrine states that any country has the right to defend itself from unwanted outside forces.