In countries that have been great powers, ideas of lost national glory are emotional resources for the nationalists, populists and conspiracy theorists of the right. They provide a sense of betrayal: We were once great, but our own leaders sold us out. They provoke feelings of humiliation: We were once the envy of the world, but now others exploit and laugh at us. And they create fears of vulnerability: We have known what it is to be strong, and now our weakness puts us in danger.

While the United States is still the world’s superpower, it does not have the same economic and military dominance it once enjoyed. Americans have experienced national humiliations in Vietnam and Iraq. Middle-class wealth and security is not what it used to be. Many voters found President Trump refreshingly honest when he claimed, “America doesn’t win anymore.”

Mr. Trump links the economic struggles of Americans with their country’s loss of standing. He says America is being exploited by its own allies and trading partners, aided and abetted by a corrupt political class. He blames immigrants for the country’s problems. America, in Mr. Trump’s telling, is always the victim.