What caused the Monroe Doctrine?

Its objective was to declare that the New World and the Old World to be separate entities. It opposed European colonialism in the new world. The Latin American countries were gaining independence from Spain and Portugal. The US was worried there would be new attempts by Old World countries to try and interfere with or try to conquer the newly independent territories and it would become a battleground. The doctrine was declared this. wouldn't be tolerated. The New world wanted to be left alone to exert its influence.

The threat of France possibly spreading the its monarchial government was a catalyst

On December 2, 1823, President James Monroe used his annual message to Congress for a bold assertion: ‘The American continents. are henceforth not to be considered as subjects for future colonization by any European powers.’ Along with such other statements as George Washington’s Farewell Address and John Hay’s Open Door notes regarding China, this ‘Monroe Doctrine’ became a cornerstone of American foreign policy.