“That’s not who we are,” Barack Obama often says when appealing to Americans to oppose illiberal policies such as torturing prisoners, barring immigrants on the basis of their religion, and denying entry to refugees. But now that Americans have elected a president who has called for precisely those policies, Obama’s confidence about who we are may seem misplaced. Questions about the defining values of our common nationality have haunted us before at critical moments in American history, and now they do again: In the wake of Donald Trump’s election, what does it mean to be an American? Will Trump and...