Short answers, pulled from the story.
John L. O'Sullivan wrote an article in 1845 that first used the phrase manifest destiny to describe American expansion. The Democratic Review published this essay titled Annexation on the 27th of December 1845.
Historian William Earl Weeks identified three basic tenets behind the concept of manifest destiny. The first assumption held the unique moral virtue of the United States above all other nations, the second assertion claimed a mission to redeem the world by spreading republican government and the American way of life, and the third faith believed in the nation's divinely ordained destiny to succeed in this global mission.
The Mexican-American War began on the 24th of April 1846 after Polk moved to occupy territory claimed by both nations. The Mexican Cession added Alta California and Nuevo México to the United States after the war ended in 1848.
Andrew Jackson signed the Indian Removal Act in 1830 setting apart an ample district west of the Mississippi River. The population of bison was reduced to a few hundreds due to intense hunting and logging while continuous plowing made soil vulnerable to erosion leading to environmental degradation that played a key role in the Dust Bowl of the 1930s.
Donald Trump became the first president to use the phrase manifest destiny during an inaugural address in 2025 declaring extension of American influence into the stars.