Chapter 6 introduces the figures and cultural forces behind the Mexican-American War, western expansion, and the concept of Manifest Destiny. The war with Mexico was started on thin pretense by President James K. Polk, who wanted above all to expand the borders of the U.S. from coast to coast. With the European imperial powers squabbling over the Americas, Polk argued that the U.S. had an opportunity to acquire as much territory as possible. He sent Zachary Taylor to disputed territory in Texas to provoke an attack; Taylor succeeded, launching the Mexican-American War.

A sickly and misanthropic man, Polk promised to limit his presidency to one term, a promise he kept. “Despite his insufferable personality,” Sides writes, “[Polk] was possibly the most effective president in American history—and likely the least corrupt” (79-80). Polk’s agenda championed a broader cultural movement at the time: American exceptionalism, an “almost religious assurance” in the superiority of the American way and the inevitability of America’s expansion to the Pacific. A New York editor named John O’Sullivan coined the phrase most often associated with this viewpoint: Manifest Destiny. Its proponents, like the Missouri Senator Thomas Hart Benton, “believed in the ruddy rightness of American power and American ideals and, especially, American commerce” (86).