The correct answer is European threats to take over Latin American colonies.
The Monroe Doctrine became one of the most famous ideas of the 19th century. This doctrine would shape American foreign policy for roughly a century.
The Monroe Doctrine stated that the Western hemisphere was off limits to further European colonization. The US promised not to interfer with already established colonies. However, any new attempt to create a colony in the Western hemisphere would be viewed as an aggressive act. According to Monroe, the US would respond with military force if needed.
This helped to put America on a path of isolationism that would continue throughout the 19th century.