How It Became

It did not become known as the Monroe Doctrine until about 30 years after it was proclaimed. Back in 1823, the United States did not hold a powerful enough government that they could not enforce Monroe's proclamation.

It did not become until the early 1900's when the United States had a growth in power and Monroe's proposal became the foundation of the nations' foreign policy.

WHO IS MONROE:

James Monroe was the fifth president of the United States from years (1817-1825).

He is best known for establishing his foreign policy principle that later became known as the 'Monroe Doctrine".

Before he became president, he spent many years in public service--domestic and overseas. He also was the first president to be a United States senator. Lastly, Monroe was one of the very honorable presidents that signed the Declaration of Independence.

http://www.americaslibrary.gov/aa/monroe/aa_monroe_doctrine_3.html