AMERICA’S FOREIGN-POLICY difficulties are multiplying, from Asia to the Middle East. Faced with the prospect of losing in Afghanistan, the president on the recommendation of his military advisers (and reversing a previous stand) has announced a new, notably vague and apparently open-ended “strategy” that includes sending additional U.S. troops. And he promises to “win,” without really explaining how we will know if we have won.

In what has been called a “silent surge,” the administration is deepening American involvement in the endless conflicts raging across the Middle East and Africa. Relations with Russia have soured measurably, if not yet to the extent Trump’s hyperbolic rhetoric suggests. A full-fledged crisis with North Korea erupted in August and continues to smolder. And the president has even tossed about possible military intervention in Venezuela, an idea that anyone with even a scant knowledge of the history of U.S. relations with Latin America would not seriously entertain.

Coincidentally, we are “commemorating” the fiftieth anniversary of the Vietnam War, a conflict that dragged on for years and tore this nation apart. Throughout 2017, the New York Times online edition ran two articles a week on the war in 1967. Ken Burns and Lynn Novick’s television extravaganza aired in September, and a flood of new Vietnam-related books is hitting the market. Such significant anniversaries, particularly those involving national calamities, understandably invite reflection, in the hope that they may aid us in avoiding a repetition of the original error.

Each historical situation is unique; to extract “lessons” from one to apply to another can be at best misleading, and at worst disastrous. The so-called Manchuria/Munich analogy that helped draw the United States into Vietnam is a prime example. Former Kennedy adviser James Thomson once proclaimed, in reductionist words—containing a large grain of truth—that the central lesson from Vietnam should be never again to “take on the job of trying to defeat a nationalist anti-colonial movement under indigenous communist control in former French Indochina,” a lesson, he quickly—and redundantly—added, that was of “less than universal relevance.” History can be a treacherous teacher. Still, an understanding of some of the things that went wrong in our once-longest war (Vietnam) might help us deal with its successor (Afghanistan) and the myriad other problems we now face.

FROM EISENHOWER through Nixon, each new administration, certain that it was smarter and tougher, took power believing it could succeed in Vietnam where its predecessors had failed. We call this the certitude of the new guys. “We will not make the same old mistakes,” Henry Kissinger confidently announced upon taking office. “We will make our own.” In fact, like those who came before, the Nixon administration did make the same old mistakes—as well as plenty of new ones.

Trump personifies, and takes to another level, this new-guy mentality. He constantly bemoans the problems he inherited from the alleged failures of those incompetents who preceded him while, often with bombastic language, he promises to solve the problems they couldn’t.

It’s a little late for this one, perhaps, except that new guys continue to go through the revolving door that is the Trump White House. These newbies, and the president himself, no matter how smart and tough they think themselves, would do well to approach the problems they confront with humility and caution, especially those, like the president, who are totally lacking in foreign-policy experience and expertise. To be sure, they should learn from their predecessors’ mistakes, but they should keep in mind that they will make their own as well.

Another lesson from Vietnam so obvious as to be self-evident—easy in, not-so-easy out—has been ignored by perennially hubristic Americans from James Madison to George W. Bush. No one has stated it better than Lyndon B. Johnson, a year before he plunged into the quagmire by drastically increasing the number of U.S. combat troops in Vietnam. “It’s damn easy to get into a war,” LBJ told National Security Advisor McGeorge Bundy prophetically in May 1964, “but it’s going to be awful hard to ever extricate yourself if you get in.”

Given how frequently neoconservatives quote Winston Churchill in so many other contexts, it is striking that they don’t take to heart this particular admonition of his: “The Statesman who yields to war fever must realise that once the signal is given, he is no longer the master of policy but the slave of unforeseeable and uncontrollable events.” Former president Barack Obama took a lot of flak for winding down the U.S. military presence in Iraq in 2010. But given how hard it is to disengage from a major military commitment, the fact that he was able to do so at all is remarkable. The lingering U.S. commitment to Afghanistan, which President Trump now proposes to increase again, is more typical of the general pattern of easy in, not-so-easy out for major military commitments, even ones that are clearly not going to end well no matter how long we stay the course.

A THIRD lesson might be called the impotence of idle threats. Inasmuch as Richard Nixon had a “secret plan” for peace, as he claimed during the 1968 campaign, it was based on the so-called “madman theory,” a stratagem of persuading an enemy of one’s irrationality and then threatening the use of nuclear weapons to force a settlement. “They’ll believe any threat of force Nixon makes because it’s Nixon,” the candidate confidently told aide Bob Haldeman while strolling on the beach.

“We’ll just slip the word to them that, ‘for God’s sake, you know Nixon’s obsessed about Communism . . . and he has his hand on the nuclear button’—and Ho Chi Minh himself will be in Paris in two days begging for peace.”

Nixon’s madman approach drew upon the mistaken belief, an article of faith among some Republicans, that Eisenhower had forced the Chinese to settle in Korea in 1953 by making nuclear threats. It also derived from academic studies of “diplomatic blackmail” developed by the economist Thomas Schelling and, ironically, by Nixon’s future nemesis, Daniel Ellsberg.

Nixon’s madman diplomacy failed. Through various sources, he warned Hanoi in the summer and fall of 1969 that if a settlement were not reached by November 1 he would resort to “measures of great consequence and force.” His threats were singularly ill timed. Ho Chi Minh died in September, and the hard-liners who had long since supplanted him were not about to dishonor his cause by giving in to the United States. Those Nixon officials tasked to devise a military program to coerce North Vietnam concluded that nothing was likely to work, and that drastic escalation would bear a high domestic political cost.

It would be natural for anyone assuming the American presidency to be awed by our military power. It is human nature to believe that small nations, even ones with nuclear weapons, can be easily cowed by Mr. Big. In fact, as various presidents have discovered, there are numerous powerful internal and external constraints on the use of the nation’s vast power. And hard experience should have taught us by now that other nations and peoples, however small and seemingly weak, are not easily intimidated.

Another cautionary “lesson” from Vietnam is to resist the temptation of quick-fix strategies—in football lingo, the Hail Mary pass. In the run-up to massive U.S. involvement in Vietnam, decisionmakers like Assistant Secretary of Defense John McNaughton, already doubtful that military intervention would compel North Vietnam to stop aiding its southern allies, sought a Hail Mary pass in the theories of coercive bombing developed by Schelling, his friend and former Harvard colleague. In late 1964, working with the State Department, McNaughton devised a bombing campaign of graduated pressures against North Vietnam that he hoped would achieve U.S. goals and avert a disaster in Vietnam. “What we need is a theory that will limit our role,” he confided to his diary.

Schelling’s model and the Rolling Thunder air campaign that drew upon it erroneously assumed the same cost/benefit calculations on both sides. In fact, the United States sought merely to demonstrate its resolve to allies and enemies; North Vietnam sought to liberate what it considered its homeland. Rolling Thunder evolved into a prolonged, vastly destructive and ultimately futile exercise in coercion.

The Trump administration, similarly, faces a set of bad choices in Afghanistan. Setting aside his own well-founded instincts against escalation, the president was persuaded by his top advisers that a hasty withdrawal would create a vacuum for terrorists such as ISIS and Al Qaeda. Like McNaughton, and Trump’s predecessor, Barack Obama, he succumbed to the lure of the quick fix in the graveyard of empires. Hail Mary passes rarely win football games. Why should we expect them to win wars?

IN VIETNAM, both U.S. president Lyndon Johnson and his North Vietnamese counterpart, party general secretary Le Duan, repeatedly miscalculated how each would respond to the other’s initiatives. In escalating the war in 1965, Johnson and his advisers assumed—incorrectly—that North Vietnam would acquiesce to an independent South Vietnam rather than risk destruction at the hands of America’s vast military power. In 1964, and again in 1968 and 1972, the aggressive—and reckless—Le Duan assumed that he could win in South Vietnam by drastically escalating the war. In each case he failed, and in so doing he imposed horrendous costs on his people and nation.