During U.S. President James Monroe's message to Congress on December 2, 1823, he stated that North Central and South America (the American continents) were no longer open for colonization by European powers and that the U.S. would view with displeasure any European intervention in the Americas. Later President James K. Polk used this doctrine to justify his ranting about Canada, Oregon, Yucatan, California and so on.
Conversely, President Teddy Roosevelt stated in 1904 that any disturbances in Latin America might result in U.S. intervention to preclude European action. This apparent duality was seen by some as an excuse for U.S. imperialism in the region.
Also contained in the doctrine was a promise that the U.S. would not interfere in wars or internal affairs between European powers. This was the source
of much of the U.S. reluctance to join WWI.