Donald Trump is the result of a fracturing, dysfunctional polity whose leaders have lost credibility. Removing the man will not change the forces that created him. Our times are Lincoln’s. For decades, politics attempted to accommodate the “peculiar institution” with individual liberty. The Missouri Compromise, the Compromise of 1850, the Kansas-Nebraska Act, all attempted to reconcile irreconcilable ends. But the conflict could not be resolved within the system. Bayonets accomplished what the Constitution could not. Perhaps no contemporary issue can be compared to slavery. But, taken together, the multitude and significance of our present differences at least equal those of...