The most dominant interest of the time was Manifest Destiny, or the social belief at the time that America was destined by God to expand from coast to coast. President Polk provoked a war with Mexico so that, by winning, he could force them to cede 40% of their country and, most importantly, California, to the United States.
This added a lot of territory that we then had to decide whether to let legal slavery in, and this split the country and the political parties, and ultimately led to a spiral of tension and even violence that would bring us to Civil War.
President Polk had already stated that the US "Manifest its Destiny" and occupy America to the Pacific. We had territory out west at the time. I believe in Califinia and definitely in the Washington state region where we threatned war with Britain if they didn't give us land up to a certain degree of latitude. There was a slogan that stated the coordinates of the area or fight. "30/60 or Fight". something like that.
The US got into a dispute with Mexico over what portion of Texas belonged to the US and that's what lead to war. Mexico sent troops to the region, but of course the big bad US gets accused of being imperialistic. Everyone fails to realize the thing to do back then was to be expansive and conquer. Much like Spain who originally took the land from the naitves that the US gained in 1847.
Our expansion lead to the end of slavery, bu that's another story :)