In the decade before the onset of the Civil War, groups of Americans engaged in a series of longshot -- and illegal -- forays into Mexico, Central America, Cuba, and other countries, in hopes of taking them over. Known as military filibusters, the goal was to seize territory to create new independent fiefdoms, which would ultimately be annexed by the still-growing United States. Most failed miserably. William Walker was the outlier. Short, slender,...