Manifest destiny was a term used by americans to represent the idea that the united states was to expand its land holdings across north america. the term was first used by john o'sullivan when the u. s. claimed the oregon territory. people who supported the idea of manifest destiny were known as expansionists. expansionists justified u. s. expansion by claiming that they were spreading democracy. from the early 1800s to the 1850s, the u. s. nearly tripled in size as gained land west of the mississippi river.