Slavery, it seemed to them, was the only firm foundation for republican government. More generally still, their peculiar institution was the unique basis of the particular outlook, assumptions, norms, habits, and relationships that defined their world and to which they had become deeply and reflexively attached. This basic difference gave rise to a protracted conflict that waxed and waned in intensity between the Revolution and the Civil War. Various aspects of that issue became the foci of that conflict at various times—including the way to apportion representation in Congress, the right to petition Congress, the right of states to nullify federal laws, and the recapture of runaway slaves. But the most persistent and explosive of issue was that of slavery’s geographical expansion. Supporters and opponents of slavery both believed that the institution needed to spread in order to survive. Slave-based agriculture was intensive and exhausted the soil quickly. It therefore constantly required additional lands. As large portions of the US population moved westward, only the creation of new slave states could sustain the slaveholders’ political power in Congress and the Electoral College. And conversely, allowing the territories, and the states carved out of them, to banish slavery would provide slaves contemplating escape new sanctuaries toward which to