The most important aspect of the monroe doctrine (1823) was that it

 showed the US backed the independence of Latin America.

Explanation:

The United States was still young at the time the Monroe Doctrine was declared, and did not have a powerful navy to be patrolling the South American coast at that time.  But the US did want to keep European powers from encroaching into the Western Hemisphere, and wanted to put Europe on notice to that effect.  

President James Monroe asserted the doctrine in his annual address to Congress in 1823.  The doctrine was that the US would not interfere in European affairs, but also would view any attempts by European powers to take control of any nation in the Western Hemisphere as a hostile act against the United States.

As reported by the US Office of the Historian, there were some additional motives in mind in the US position, in addition to backing the independence of Latin American nations.  "Monroe’s administration forewarned the imperial European powers against interfering in the affairs of the newly independent Latin American states or potential United States territories. While Americans generally objected to European colonies in the New World, they also desired to increase United States influence and trading ties throughout the region to their south."

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