Monroe why would the US use this as a policy for relationships with other countries

On this day in 1823, President James Monroe delivers his annual message to Congress and calls for a bold new approach to American foreign policy that eventually became known as the "Monroe Doctrine." Monroe told Congress, and the world’s empires, that "the American continents are henceforth not to be considered as subjects for further colonization by any European powers." This policy was invoked and adapted by subsequent presidents to advance American economic and political interests in the Western Hemisphere.

Monroe’s declaration, which was drafted by Secretary of State John Quincy Adams-who would succeed Monroe as president in 1824-was aimed at preventing attempts by other nations to colonize territory on the North and South American continents that had not yet been claimed by Europeans. Although the U. S. population was at the time concentrated east of the Mississippi River, expansion into the western half of the continent was foremost in the minds of many American politicians, including Monroe and his predecessor Thomas Jefferson. Monroe and Adams were also concerned that the British, French and Russians would attempt to annex regions once held by the Spanish (such as the Southwest, Central and South America and the Northern Pacific) - places over which the U. S. itself hoped to extend control.