"Our country has entered on a new epoch of its history," wrote a Whig Party journal in 1849, just after America's triumph in the Mexican War. Indeed, for that romantic generation of Americans in the mid-nineteenth century, the Mexican War was a grand exercise in self-identity: it legitimized the young republic's convictions of mission and destiny to a doubting world. It was easily one of the most popular wars the United States has ever fought. This...