In June, Teddy Fischer, a reporter for a high school newspaper in Mercer Island, Washington, scored an interview with Defense Secretary James Mattis. During that interview, Mattis remarked that, no matter what you want to do, “I don’t think you can go wrong if you maintain an avid interest in history.”

I agree. But there’s good history, and there’s not-so-good history. Fortunately, there’s at least one way to tell them apart. The tip off: simplicity.

Mattis did note that some simple things, such as the fundamental nature of war, do endure. To understand this, he recommends Thucydides’ “The History of the Peleponnesian War.” Thucydides said that the fundamental nature of war is “fear and honor and interest,” and Mattis noted, “those continue to this day.”

But, Mattis added, when it comes to the way wars are fought, “they’re like chameleons, a dead German guy wrote.”

By not naming the dead guy, Mattis was keeping it light. He was referring to Carl von Clausewitz, who, in his classic “On War,” wrote that, “War is more than a true chameleon.” Mattis knows something of war, and he knows his Clausewitz, too.

Or at least he knows it as well as anyone can, because there’s enough in Clausewitz to keep you busy for a lifetime. Many great scholars, from Sir Michael Howard to Christopher Bassford, have sought to understand Clausewitz’s insights into the nature of war.

The most successful of these efforts is Jon Sumida’s 2008, “Decoding Clausewitz.” It’s not exactly an easy read, but Sumida makes it as easy as it can be. His method is easy to describe, if not to practice: Sumida assumes Clausewitz understood what he was writing, and then figures out what he meant.

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I studied with Sir Michael, and knew Sumida. They’re among the smartest men I’ve ever met. And Clausewitz wasn’t trying to be obscure. But he was writing about war, and that’s hard. History — real history — is complicated.

Take Clausewitz’s assertion that war is a continuation of politics by other means. Everyone knows this phrase, even if they don’t know where it comes from. But as Sumida points out, this isn’t, on its own, much of an insight. Intuitively, we know war is a struggle between adversaries with political differences.

But the point of Clausewitz isn’t his famous phrase. Sumida concludes that Clausewitz’s argument is that in war, politics affect everyone, but especially the attacker. Hence, the defense — not the attack — is the stronger form of war.

The value of understanding the classics occurred to me when considering Graham Allison’s “Destined for War: America, China, and Thucydides’s Trap.” It’s now so popular that when you search for Thucydides, it’s the second suggestion on Google.

Allison is a titan in the study of international relations: his “Essence of Decision,” published in 1971, is still a must-read. “Destined for War” is not.

As Arthur Waldron, professor of international relations at the University of Pennsylvania, points out, Allison’s thesis rests on a lapidary sentence in Thucydides — that the rise of Athenian power made Sparta fearful and, thus, made war inevitable. Like Clausewitz’s famous phrase, this is memorable stuff.

It’s also wrong. Thucydides’ history shows the war wasn’t caused by Sparta’s fear of Athens’ rise. It was caused by Athens, which wanted to cement its dominance. So Allison’s thesis — that the United States is fated to attack China, the rising power — isn’t based on what Thucydides wrote.

It turns out that predicting the future by leaning on a famous phrase isn’t enough to make a history good. Indeed, it may be enough to make it bad. I’d recommend you read Sumida and avoid Allison. But read what you like. Just remember: if it makes history seem simple, it’s probably not good.

Ted R. Bromund is a senior research fellow in The Heritage Foundation’s Thatcher Center for Freedom.