Barton Swaim is the author of "The Speechwriter: A Brief Education in Politics" and a contributing columnist at The Washington Post.

President-elect Donald Trump in a meeting with technology executives at Trump Tower on Dec. 14. (Drew Angerer/Getty Images)

In 1987, Politico reported this week, former president Richard Nixon wrote a brief letter to future president Donald Trump. “Mrs. Nixon told me that you were great on the Donahue Show,” Nixon wrote. “As you can imagine, she is an expert on politics and she predicts that whenever you decide to run for office you will be a winner!” The letter will be displayed in the Oval Office.

I am not aware that Trump’s interest in Nixon extends beyond this letter, and in any case comparisons between the 45th and 37th presidents wouldn’t seem to go far beyond a couple of general tendencies: Trump’s hatred for the Washington press corps strikes one as Nixonian, and of course neither man will be remembered for ideological consistency.

The two men’s styles may converge at one significant point, however, on the matter of foreign relations. Like Trump, Nixon was concerned to keep his adversaries guessing about his motives and temperament. Nixon never wanted the Soviets to feel confident that they knew what he would do, or that he wouldn’t do something outrageous or irrational. His famous term for this tactic, recalled by H.R. Haldeman in his posthumously published diary, was the “madman theory.” Nixon wanted the North Vietnamese, with whom the United States was negotiating a peace agreement, to feel a sense of apprehension about what the president might do if pushed to the brink. Recalled Haldeman:

We were walking along a foggy beach after a long day of speechwriting. He said, “I call it the Madman Theory, Bob. I want the North Vietnamese to believe I’ve reached the point where I might do anything to stop the war. We’ll just slip the word to them that, for God’s sake, you know Nixon is obsessed about Communism. We can’t restrain him when he’s angry — and he has his hand on the nuclear button — and Ho Chi Minh himself will be in Paris in two days begging for peace.”

And Nixon did more than “just slip the word.” In October 1969 he dramatically raised the Strategic Air Command’s readiness level and made a nuclear strike look imminent, evidently in an attempt to rattle Ho’s Soviet backers. Melvin Laird, Nixon’s secretary of defense, later observed that this was done to make the Russians think “you could never put your finger on what he [Nixon] might do next.”

Put to one side your views of Nixon, the Vietnam War, and the way peace terms were negotiated from 1968 to 1973. At least this much is true: In international negotiations, particularly with distrustful or belligerent counterparts, the ploy of making the other side believe you can’t be pushed too far without inviting terrible consequences — or at least that your decision-making can’t be easily predicted — might occasionally have its uses.

One of the challenges any American president has vis-à-vis heads of undemocratic regimes is just this, that whereas the autocrat’s motives must oftentimes be guessed at, an elected president has by definition spent months openly enunciating his views about America’s role in the world. Nixon tried to compensate for that disadvantage by hinting through intermediaries (Henry Kissinger and others) that he might be emotionally or psychologically unstable. Somehow I doubt it worked, or anyhow I doubt it worked well: Everything about Richard Nixon suggested cold calculation and calmly reasoned strategizing — he might get angry but he wasn’t going to start a war in a fit of rage — and in any case Russian diplomats would have assumed some component of duplicity in any message delivered by their American counterparts, however seemingly candid.

Donald Trump is a different matter altogether. He doesn’t need to have intermediaries suggest by furtive murmurs that the boss might be a little crazy. Everybody thinks that already.

It’s true, as Post columnist Dana Milbank contended recently, that Trump’s studied unpredictability may have dire consequences at home and abroad. Milbank wrote: “In Trump’s application of the Madman Theory there seems to be less theory than madman. There may be advantages to keeping foes and opponents off guard, but Trump is baffling friends and allies, too. In foreign affairs, unpredictability spooks allies and spreads instability.” Maybe. But we’ve had many a “spooked” ally before, and oftentimes a spooked ally is still very much an ally. As for instability, we’ve had plenty of it under the highly predictable Obama administration.

So put to one side your views of Trump, his character, his statements — and consider the possibility that he may be more calculating and deliberate than anybody gives him credit for. Nixon’s madman theory may in that case prove more effective for Trump than it ever did for Nixon. Even after hearing Trump talk about nearly everything for 18 months, no one’s sure what he’ll do or even what general approach he’ll take to foreign confrontations. If there is a foreign leader who feels he or she has got the measure of President-elect Trump, that foreign leader is an idiot.

Whether Trump uses the advantage of unpredictability for good or ill is anybody’s guess. There are grounds for thinking he may pursue a shortsighted America-first realpolitik, and in that case the agreements he strikes with other nations will benefit American industry and interests in the narrowest sense.

But events have a way of upending presidents’ worldviews — George W. Bush, remember, rejected “nation building” as a presidential candidate — and it’s just conceivable that Trump may use his cunning to accomplish worthier goals than his myriad critics think him inclined to pursue. Trump, in other words — the madman, not in theory, but in practice — may bring off some creditable diplomatic victories precisely because foreign leaders and their emissaries don’t feel it’s in their interest to test the White House. It’s hard to imagine, for instance, President Trump’s secretary of state going cap in hand to the Iranians even after the Iranians have failed, or refused, to uphold their end of a painstakingly forged agreement. Hard, too, to imagine a Syrian regime deliberately crossing a red line just to see what President Trump will do.

At this point, though, we have no idea what he’ll do. And that’s the point.