Much of the impetus for westward movement came from Southern famers, who customarily moved when the soil they farmed became less productive, or about once every twenty years. A typical family pattern starting about 1800 will take people from the central Carolinas west to Tennessee or Kentucky, then south to Mississippi or Alabama. From there family members might leave for one of the Southern states west of the Mississippi. By 1860 they could have been in east Texas for several years and thinking about moving to a new frontier.
An early chapter in James Oakes book, The Ruling Race: A History of American Slaveholders and Slavery, describes small slaveholders (less than twenty slaves) as men always thinking about new land and better opportunities. New land included not only territories of the American West, but also the islands of the Caribbean and Central America. This should explain Southern interest in Nicaragua and Cuba during the late 1850s.
Oakes counters the main thesis of the Marxist historian Eugene Genovese, who argued that slavery was unprofitable. According to Genovese, slaveholders maintained the institution as a means to control their world -- government, society and economics. This is difficult for Genovese to explain because race has no significance within the Marxist interpretation of history. Race formed the basis of Southern social hierarchy, not class as the Marxists would have it. Oakes produces a wide variety of original sources to support his contentions, another being that slaveholders were agrarian capitalists, highly motivated to accumulate wealth and material possessions.