In this lecture from Yale's America at 250: A History course, Professor David Blight opens his section of lectures with an in depth review of the Mexican American War of 1846 to 1848. He examines how the war reshaped the territorial map of the United States and how the vast new lands it brought under American control intensified the political conflict over the expansion of slavery. The lecture situates the war within the broader arc of nineteenth century American expansion and sets up the sectional tensions that would dominate the following decade. Recorded in Yale's Sprague Hall as part of the DeVane Lecture series, the course traces United States history from 1776 to the present through the perspectives of three Yale historians.