If Donald Trump were not (alas) so ignorant, he would envy the quality of Joe Biden’s inaugural speech pronounced earlier today. But there is a deep question to ask: Why are political rulers so insistent on “unity.” It was the main theme of Biden’s speech, where the word appears eight times. It was also a constant theme with Trump—but muffled as time went on. Remember his remarkable 2016 campaign ad, which is well worth listening to: I will unify and...