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I lack the authority to pronounce grandiosity a mental disorder, but I do know it can be dangerous — especially to the subjects of a ruler who exempts himself from human fallibility.

So it was disconcerting to realize I was regularly glancing at my cellphone’s screen while reading “Fire and Fury: Inside The Trump White House.” The president has conditioned me to expect a regular flow of news about his claims to being the world’s greatest something.

Sure enough, as I reached Chapter 2, Trump said he understood the new tax code better than any CPA. By Chapter 5, he proclaimed himself a “very stable genius.”

Before I finished the book, he’d told an audience of farmers who voted for him: “You are so lucky I gave you that privilege” — though generally that privilege is credited to the U.S. Constitution. Trump previously claimed to know more about ISIS than the generals.

Now it’s just not normal to claim first place in so many fields. Muhammad Ali said: “I’m the greatest!” But he didn’t invite fans to join him at a blackboard while he solved partial differential equations.

Yet whatever the subject, Trump is quick to say he’s better than best.

Recently he noted that fewer jets have crashed since he’d become president. What could possibly link those phenomena?

Some critics say it’s delusions of grandeur — a conclusion most psychiatrists decline to endorse. Their professional association says it’s unethical to diagnose someone you haven’t examined.

But I’m a historian, a member of a profession that has to pass judgment on people they’ve never seen. That is what we’re trained to do. Suppose a student had asked what made Julius Caesar tick, and I replied, “Sorry, I never met him.” The dean would’ve canceled my paycheck.

Historians have an advantage over the shrinks, who deal in causes — Oedipal fixations, stuff like that — that tell us how a person got to where he is. Anxious Americans want to know what Trump might do next. Lacking a professional jargon, a historian can answer that question much like a racetrack handicapper: by allowing Americans to compare Trump’s grandiosity with the inflated egos of the past.

So let’s provide word pictures of rulers whose narcissism brought disaster upon their people — or did some good.

Hitler thought he knew infinitely more than his generals. So he took on Russia, without considering that the Kaiser had tried that in World War I and that Napoleon’s army was destroyed there in 1812. Hitler’s troops didn’t pack warm clothing because he was convinced they’d be in Moscow before winter. They suffered 680,000 casualties in three months, then the snow began to fall.

How could Stalin think anything other than that he was a genius? Soviets who didn’t say so were sent to the gulag. While Hitler was conquering other nations, Stalin signed a nonaggression pact with him. When British intelligence warned that his turn had come, Stalin wouldn’t listen. His troops were taken by surprise by Hitler’s tanks, and some 20 million Soviets perished in World War II.

Joan of Arc was convinced that God had sent her to crown a Frenchman king of France. Her mistake was to say so within hearing of the English who had come to think of France as theirs. So Joan was burned at the stake — but her idea lived on. Frenchmen decided the time had come to kick the foreigners out, which is why at breakfast in Paris, it’s easier to find a croissant than an English muffin.

Yet in 1870, it was tough to find anything to eat in Paris, thanks to Napoleon III. A nephew of Napoleon, he thought the blood of military genius flowed in his veins. But he was snookered into picking a fight with the Prussians. And while military cadets routinely learn the mantra “always take the high ground,” Napoleon III marched his troops into a valley surrounded by mountains. Artillery shells rained down on the French.

So sharpen your pencils, compare Trump with those examples, and mark your score cards: Is he dangerous, tolerably odd or a genius?

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