Volume 12 of the Correspondence of James K. Polk documents a critical seven months in one of America's most transformational presidencies. Polk was the eleventh U.S. president (1845-49). Many of this volume's letters chronicle the Tennessean's prosecution of the Mexican War, a conflict that, along with his 1846 acquisition of what is today's Pacific Northwest, increased by one-third the size of the United States. The letters, most of them until...