Excerpts from Books and Wikipedia
"In his national message addressed to Congress, President Monroe declared that, 'the American continents, by the free and independent condition which they have assumed and maintained, or henceforth not to be considered as subjects for future colonization by any European powers.'. . . The Monroe Doctrine had little importance until later in the century. It was the British Navy, not the United States, that kept South America independent." [Furtado: 1001 Days] "In terms of international power politics, the Monroe Doctrine represented the moment when the United States felt strong enough to assert a 'sphere of influence' that other powers must respect. . . . The United States seriously invoked the Monroe doctrine for the first time only after the Civil War, when it persuaded Napoleon III to withdraw French military support from Maximilian von Habsburg in Mexico. Thereafter the doctrine loomed increasingly large in the American public imagination." [Howe: What Hath God Wrought, p. 111-16]