Which of these had the greatest impact in the u. s fulfilling it's manifest destiny

American expansion to the west that was destined to stretch from coast to coast. American culture was changed during Manifest Destiny because when the Americans came in to Texas to take their land. The Americans were upset with the Texan's culture, so they tried to change it. This caused Mexico to attack the Americans.

the monroe doctrine is the best known u.s. policy toward the western hemisphere. buried in a routine annual message delivered to congress by president james monroe in december 1823, the doctrine warns european nations that the united states would not tolerate further colonization or puppet monarchs.