Unity eludes Americans because we have forgotten what it means. Americans in our time feel divided and estranged, and yearn to be more unified. Yet we have lost sight of what unity involves in a complex free society. We think it’s both easier and harder than it really is — imagining that greater cohesion could quickly solve our biggest problems, but that it would require us to put aside our disagreements. This is wrong on both counts. Unity requires constant work toward negotiated...