Answer:The surge of pride in America at the time of Manifest Destiny pushed the expansion of the US into the western territories and lead to the annexation of Texas, all of Mexico's western lands, California and more. ... Politics, fear of war with Mexico and fear of another country getting them.
Explanation:
The government promised settlers grants of land and money
Pretty sure it is "gold", you know with the California Gold Rush, and 49ers and all that. :)
The large land was isolated from the rest of the world by deserts oceans and mountains which enabled them to develop independently from other civilizations