America's war with Mexico was one part of a broader expansionist movement encouraged by the concept of manifest destiny. Therefore, to understand the war, your students will need to have a broader understanding of America and its purported mission.  

Most Americans at the time supported this philosophy of expansion at some level, but many also recognized certain problems associated with it. Therefore you should take this opportunity to explore the complexities of expansion—the concerns about racial mixing in Mexico, the sorts of methods by which expansion should be accomplished, and whether slavery should be a part of America's movement westward.