Words matter. Especially when uttered by a president, and especially overseas. “Speak softly, and carry a big stick,” Theodore Roosevelt advised, though he never envisioned a successor would prove capable of obliterating cities half a world away in under half an hour. That nuclear stick is pretty big indeed, capable since 1945 of keeping our most virulent adversaries, including Moscow, Beijing, Tehran and Pyongyang, from their most reckless ambitions. It also keeps allies in line. What do Japan, Saudi Arabia, Germany and South Korea have in common? Each is but a day away from joining the nuclear club. That day is when their leaders stop believing the president of the United States will come to their aid.

This is why I fear a second Trump term. A world increasingly riven by renewed great power rivalries and historic animosities is further weakened by Oval Office instability, exemplified by ill-advised remarks, ill-timed threats and outright lies. Calm captains of the ship of state struggle to navigate the world system’s waves and shoals. An erratic one won’t help. Especially one whose obsessions, personal grievances and loose relationship with the truth make others question not only America’s policy but more fundamentally our reliability.

How trite. The professor in the ivory tower reminds us that words retain meaning. How very 20th century. Does he not realize that legions of bots and ChatGPT enable today’s policymakers to forge the algorithmic reality they desire?

Presidents must be held to higher standards. Their quips move markets. Their words invite or ward off aggression. Save or end lives. Examples abound of even experienced leaders forgetting their rhetorical reach.

Dwight Eisenhower’s 1956 promise of aid inspired Hungarians to revolt against Soviet control, leading most to their death or to exile. Ike never thought they’d take him so literally. He meant moral and rhetorical aid, the thoughts and prayers kind. Hungary’s freedom fighters expected guns, or better yet American troops that Eisenhower never meant to imply would be forthcoming. Desperate people heard what they wanted to hear when the man in the Oval Office was unclear.

Words mattered at the end of the Cold War, too. Ronald Reagan called the Soviet Union an “evil empire” against his own State Department’s guidance, which also tried to stop him from saying “Mr. Gorbachev, tear down this wall,” lest such a direct challenge rile the Kremlin. But that was precisely Reagan’s point. Another word for “rile” is “catalyze,” which is what Reagan hoped to do to the rumblings of change behind the Iron Curtain.

George H.W. Bush also understood the power of presidential proclamations and was thus largely mute when the Berlin Wall finally fell in 1989. “I guess I’m just not an excitable guy,” he told CBS reporter Leslie Stahl and a similarly bewildered White House press corps amazed by his laconic response. But Bush knew presidential triumphalism at that precarious moment might spark a hard-line backlash. “I’m not going to dance on the wall,” he said privately, forgoing personal political gain to preserve America’s Cold War triumph.

Presidents are supposed to care more about the nation’s fate than their own. Barack Obama’s reputation suffered when he refused to back up his own “red line” against Syrian use of chemical weapons in 2013, but he ultimately reasoned his promise to avoid another Middle East quagmire mattered more than his own temporary loss of prestige. Joe Biden’s decision to keep his promise to end America’s generation-long fight in Afghanistan showed consistency even when retaliation for losses endured during the evacuation might have helped him in the polls. Better to demonstrate prudence, he reasoned, than to rashly reverse a well-considered decision in hope of temporarily saving face.

This is why the prospect of a second Trump presidency is so terrifying: His unconsidered words reverberate. He was the first president since Harry Truman, which is to say the only president ever, to cast doubt on our commitment to defend our NATO allies. “Does that mean that you won’t protect us in case — if we don’t pay, you won’t protect us from Russia,” Trump boasted a foreign leader lamenting. “I said, ‘That’s exactly what it means.’”

Perhaps this was more bluster than extortion, a negotiating tactic to encourage tight-fisted allies to boost their defense spending. Either way, the story has become part of Trump’s standard rally repertoire. Our allies, meanwhile, inch closer to creating their own security guarantees every time Trump puts another dent in the armor of collective security. Including their own nuclear deterrent.

Treaties and promises are, ultimately, mere scraps of paper. They only matter if leaders are trusted to follow through. After a decade of undermining Washington’s commitment to NATO, including four years as president, Trump has no reservoir of reliability among our partners, at least those that remain beyond the grip of their own strongmen. During a second Trump presidency our most meaningful allies would be certain to further their own security arrangements without U.S. involvement and thus, without U.S. input. After all, would you buy a second car from a dealer that threatens to disregard the warranty on your first?

Trump’s prevarications underline his unreliability. He will say anything that leaps to mind, or anything he thinks will help him win, no matter the veracity or collateral damage. A presidential candidate willing to lie about immigrants, FEMA, military chiefs or a hurricane’s expected path can’t be trusted to tell the truth about future crises. Worse still is his tendency to double down rather than admitting error. If Trump’s putting America First means risking the well-being of Ohio schoolchildren, continuing to push the Big Lie that he won the last election or redefining the Jan. 6 Capitol assault as pure patriotism rather than partisan violence, why would our foreign friends trust his judgment?

Trump’s falsehoods are unmatched in presidential history. Franklin Roosevelt promised that Americans would build 50,000 aircraft a year to combat Nazi aggression. Asked by aides where he got that big, round number, Roosevelt responded he’d made it up, noting that defeating fascism required Americans to think in larger terms than ever before. Abraham Lincoln fibbed as well, telling newspaper readers in 1862 that he was not considering emancipating the Confederacy’s enslaved people when he’d already decided to do so. Even the greats sometimes lie, albeit for national rather than personal gain. Trump lies for himself.

International politics is not best overseen by saints or sophists. We are forced to trust the person we put in charge of our security to use their words judiciously. But Donald Trump shuns what Ike learned, Reagan deployed, Bush restrained and Obama realized: The big stick of American power requires speaking not so much softly as reliably.

FDR and Lincoln knew when they were lying. Does Trump? The world should fear another four years of wondering if he can tell the difference.

Jeffrey A. Engel is the founding director of the Center for Presidential History at Southern Methodist University. He is at work on his 15th book, “Seeking Monsters to Destroy: How Americans Go to War from George Washington to Today.”