The Mexican War definitely promoted the U.S.'s interests. The acquired territory was quite rich and was just waiting for people to exploit those riches. Within one year after the end of the war there was a gold rush at Sutter's Mill in California, and after the Civil War there were gold and silver strikes all over the West. California was also rich in ports and eventually these ports would be used to get in on the Chinese trade. Northern and central California would be valuable for their long growing seasons and great soils. This region would be known for wine, grapes, and livestock. In the twentieth century Western tourism would also be valuable to American interests.
At the time, the acquisition of the Mexican Cession proved dubious as the United States invaded disputed territory in order to bait Mexico into a war, but after the war, the acquisition was great for American nationalism. The issue of ownership in the Oregon territory was already solved, and the Mexican War gave the United States a claim to the entire continent, save for a little piece in Arizona purchased in 1853 as the Gadsden Purchase. Many expansionists were starting to talk about the United States finally being able to achieve its democratic and economic destiny.