On the dry, sun-baked afternoon of Aug. 21, 1858, Abraham Lincoln and Stephen A. Douglas stepped onto a hastily constructed platform in the main square of the Illinois River town of Ottawa and began a series of seven one-on-one debates that bored a smoldering hole straight into the heart of American politics. Both men were candidates for the U.S. Senate, and both were veterans of the conventional give-and-take of politics. But these debates would carry Lincoln and Douglas far above the conventional. They would focus on an issue that promised a whirlwind of catastrophe for America, and they would end...