"Who is James K. Polk?" was a rallying cry of the Whigs during the campaign of 1844. Polk answered that question adequately by winning the election against his Whig opponent, Henry Clay. Today the question might be recast--respectfully, not derisively--"Who was James K. Polk?" Few persons could give more than a perfunctory answer, even though when he left office the United States was half again larger than it was when he became president...