In the years leading up to the Civil War, many Northerners and Southerners alike wanted the federal government to take a more aggressive approach toward acquiring new territory. In fact, some private citizens, known as filibusters, took matters into their own hands. They raised small armies illegally; ventured into Mexico, Cuba, and South America; and attempted to seize control of the lands. One particularly successful filibuster, William Walker, actually made himself president of Nicaragua and ruled from 1856 to 1857. For the most part, these filibusters were just men in search of adventure. Others, however, were Southern imperialists who wanted...