The California Gold Rush sparked a movement west, which only further ignited manifest destiny. People saw the opportunity to stake a claim of their own and truly pursue the "American Dream" out west. This new discovery and the abundance of wealth to be had further solidified support of Polk's decision to move westward.
A primary source. That would be a source coming directly from someone who was in the war so you know it is reliable and probably the most accurate. A secondary source could be from a journalist or an outside mind that doesn't know the true experience of being in the war and is just observing from an outsiders view.
