Melly doesn't believe that troops are needed at home, because she thinks all the slaves are happy. In fact, though, there were numerous slave rebellions throughout Southern history, the most famous perhaps being Nat Turner's rebellion in 1831, in which seventy enslaved people killed about sixty white people in Southampton County, Virginia.
There were also slave revolts and attacks during the Civil War; for example, in 1864 slaves burnt fourteen houses and the courthouse of Yazoo City, Mississippi. To maintain the terrible status quo, then, there was every reason to believe that troops were needed at home. But the novel can't acknowledge this, because to do so would mean that the slaves were being kept in slavery by force, and this novel prefers to depict black people as happily oppressed.