When James K. Polk was elected president in 1844, the United States was locked in a bitter diplomatic struggle with Britain over the rich lands of the Oregon Territory, which included what is now Washington, Oregon, and Idaho. Texas, not yet part of the Union, was threatened by a more powerful Mexico. And the territories north and west of Texas - what would become California, Nevada, Utah, Arizona, New Mexico, and part of Colorado - belonged to Mexico.

When Polk relinquished office four years later, the country had grown by more than a third as all these lands were added. The continental United States as we know it today was established - facing two oceans and positioned to dominate both.

In a one-term presidency, Polk completed the story of America's Manifest Destiny - extending its territory across the continent, from sea to sea, by threatening England and manufacturing a controversial and unpopular two-year war with Mexico that Abraham Lincoln, in Congress at the time, opposed as preemptive.

Robert W. Merry tells this story through powerful debates and towering figures: the outgoing President John Tyler and Polk's great mentor, Andrew Jackson; his defeated Whig opponent, Henry Clay; two famous generals, Zachary Taylor and Winfield Scott; Secretary of State James Buchanan (who would precede Lincoln as president); Senate giants Thomas Hart Benton and Lewis Cass; Daniel Webster and John C. Calhoun; and ex-president Martin Van Buren, like Polk a Jackson protégé, but now a Polk rival.

This was a time of tremendous clashing forces. A surging antislavery sentiment was at the center of the territorial fight. The struggle between a slave-owning South and an opposing North was leading inexorably to Civil War. In a gripping narrative, Merry illuminates this crucial epoch in U.S. history.