Many remember the formula Ronald Reagan made famous in the 1980s, “Peace Through Strength.”  Like Reagan, Trump believed it and implemented it. So did Roman Emperor Hadrian, George Washington, and Theodore Roosevelt. Deterrence works, and is – at last – back!

This idea is well understood by military historians, and historians generally since core principles do not change. They do get forgotten until rediscovered. That said, this idea is key and has been in history.

Somewhere between 117 and 138 AD, the Roman Emperor Hadrian coined the phrase and used it a lot. His quote: “Seek peace through strength, or failing that, peace through threat.”

There is a difference, of course, since a threat without strength, resolve, and credibility will be ineffective, even inviting an attack, is just empty words. Meantime, wordless strength is arresting.

Hadrian himself was an interesting character, not entirely favorable, but effective when it came to putting down enemies of the Empire. How? In short, he used a three-pronged strategy.

First, he prioritized security over absolutely all else. Where other emperors put expansion and whims first, he put security in that top slot, put money and men against it, and never stopped doing that.

Second, he stayed away from what other emperors thought was fine, intervention, and expansion. To the dismay of many expansionist senators, he pulled up a proverbial draw bridge, no new wars.

Third, he worked to create a kind of missing internal security, thinking unity mattered most. Call it the rule of threes, but he used three tools for that. Believing social integration equaled security, he pushed unity of cultures. Thinking people needed their leader, he got to every province and was seen.

Finally, this “peace through strength” guy Hadrian was a born builder, rebuilt the Pantheon, built a temple to Venus and Roma, and famously – yes – built walls to protect the empire, including to the far north from Scots. Does Hadrian’s Wall ring a bell? That wall protected the province of Britania.

History is ironic. Today, what is left of “Hadrian’s Wall” is a British tourist attraction, and the peace-loving United Nations has designated Hadrian’s Wall a “World Heritage Site,” and did that under Reagan.

So, what worked and why? How did Hadrian’s version of “peace through strength” perform? Very well. He prioritized security, forgot intervention, unified people with social integration and his constant presence, and – to keep all that internal stability – built strong, protective walls.

Interesting, eh? Other leaders know the mantra and have had creative ideas, Albania ringing itself with foxholes, the French overstretched Maginot Line, perpetual territorial expansion, centralizing everything, Japan’s WWII kamikaze pilots, or terrifying the population with endless horrors.

The four most effective purveyors of “Peace Through Strength” in US history have been George Washington, Theodore Roosevelt, Ronald Reagan, and Donald Trump. Overstatement? No, true.

Washington, having fought wars,  knew how to stay out of them – and told us how. In his fifth State of the Union, 1793, he said: “If we desire to avoid insult, we must be able to repel it; if we desire to secure peace, one of the most powerful instruments of our rising prosperity, it must be known that we are at all times ready for war.” Adams and Jefferson held tight to that principle.

Later, Theodore Roosevelt – a veteran of San Juan Hill, Spanish-American War – made our defenses strong, sent the “Great White Fleet” around the world, negotiated an end to the Russo-Japanese war, and wrote 37 books, many unifying as well as entertaining.

His quotes are legion, often on point. “Don’t hit at all if it is honorably possible to avoid hitting, but never hit soft.” “A good Navy is not a provocation to war; it is the surest guarantee of peace.”

Reagan, of course, was tightly tied to that doctrine – and all that comes with it. Reagan was surefooted, neither trigger-happy nor gun shy, and sage enough to bring our enemies around, Lincoln-style, converting them with a healthy dose of persuasion – and strength behind words.

And so now, at last, we come to President Trump, who has suffered every slander known, been betrayed from within, maligned from without, yet stayed course. Already adversaries recognize the Hadrian within Trump and respect it. They know he is a man of his word, believes in “peace through strength,” but never hits soft. Deterrence works, and is – at last – back!

Robert Charles is a former Assistant Secretary of State under Colin Powell, former Reagan and Bush 41 White House staffer, attorney, and naval intelligence officer (USNR). He wrote “Narcotics and Terrorism” (2003), “Eagles and Evergreens” (2018), and is National Spokesman for AMAC.