The correct answer to this open question is the following.
The way in that the articulation of the Monroe Doctrine represented a turning point in American history was that United States President James Monroe was very clear in 1823 when he informed the US Congress that the government would practice the foreign policy of neutrality in European affairs. However, he warned European superpowers to remain to intervene in the issues of the American continent. If Europeans tried to mess with American issues, that could be seen as direct aggression to the United States.
The light that this doctrine shed on the vision of America's future shared by the doctrine's supporters was that the United States wanted to defend ist economic and political interests in North America and Latin America, in a time when European nations controlled important territories in Latin America, as was the specific case of the Spanish monarchy.
