Most Americans imagine the Civil War in terms of clear and defined boundaries of freedom and slavery: a straightforward division between the slave states of Kentucky and Missouri and the free states of Illinois, Indiana, Ohio, and Kansas. However, residents of these western border states, Abraham Lincoln's home region, had far more ambiguous identities-and contested political loyalties-than we commonly assume.

In The Rivers Ran Backward, Christopher Phillips sheds light on the fluid political cultures of the "Middle Border" states during the Civil War era. Far from forming a...