U.S. History: American Motivation for Entering the Mexican-American War
America was looking to acquire a very large territory from Mexico in the Mexican-American war. They went to war with Mexico in order to expand all the way to the Pacific Ocean, providing harbors on the west coast for trade and room for either slavery or non-slavery, depending on how the territory would be organized, to grow and dominate the nation.
One basic reason for seeking to acquire much of the territory owned by Mexico is that this would allow America to expand westward, as families were always looking to move to new areas with better opportunities.
An additional benefit of new territory, as seen by southern slavery advocates, was the chance of spreading slavery into the west to uphold the power of the slave states in the government. The size of the area that belonged to Mexico was almost that of the organized states at the time. Also, most of it was in the south, which led the southerners to believe that slavery would be permitted if the territories were acquired. As a result, they supported the war and many of them went to fight for the expansion of slavery into new territories.
In addition to spreading the spirit of democracy embodied in Manifest Destiny, the southerners wanted to spread slavery with it. The territory that could be gained in the war was enough to get the southerners excited about fighting the war so they could use the land to gain control of the Senate and House of Representatives for the non-abolitionists. Charles Sumner wrote in April of 1847 that "This was to be a war of invasion, a war of territorial conquest… It is a war for the extension of slavery over a territory which has already been purged by Mexican authority from this stain and curse. Fresh markets of human beings are to be established; further opportunities for this hateful traffic are to be opened." By getting the territory, slavery would be able to grow once again, despite the efforts of America and Mexico to outlaw it.
This, of course, is assuming that the territory would be open for slavery. Because it was not set in stone how the territory would be organized, there would also be hope in the North of gaining power in the government by creating free states out of the territory won in the war. This means that at the time of the war, everybody was interested in the territory, seeing it as a means to spread their cause.
In addition to these sectional interests, the nation as a whole had some economic interests in the land. One of these interests is the access to the west coast that it provided. If America acquired California, then they would be able to make ports there and have direct access to trade with Japan, China, and other eastern Asian countries. Since they could not buy only the strip of land along the coast, America was forced to hope for the best in their demand for all of the western land.
Andrew Cayton, America: Pathways to the Present, 2007
N.C. Brooks, A Complete History Of The Mexican War: Its Causes, Conduct, And Consequences: Comprising An Account, 1849