Now that Donald Trump is officially the president of the United States, he is in complete control of America’s nuclear arsenal. Should he decide to start a nuclear war, there are no legal safeguards to stop him. Instead, a much less tangible web of norms, taboos, and fears has reined in US presidents since World War II. But as North Korea escalates its nuclear weapons tests, Russia promises to strengthen its nuclear forces, and the new President of the United States has openly tweeted that the US must “strengthen and expand its nuclear capability,” experts worry that this fragile web could start to tear.

Let’s be very clear: the president alone controls the nukes

Trump’s position on nukes has been murky, at best. In the last few weeks, he jumped from advocating for an arms race, to implying that the US and Russia might work together to reduce nuclear proliferation.

In fact, during his campaign, he called nuclear proliferation the “biggest problem” in the world. But then he also said that Japan and South Korea might want to get nukes of their own. He wouldn’t take nuking ISIS, or even Europe, off the table. But he’s also characterized himself as “highly, highly, highly, highly unlikely” to ever use nuclear weapons. This calculated ambiguity isn’t unusual for America’s presidents. Presidents Bill Clinton and George W. Bush left nuclear first strikes on the table, too.

But for a US president to talk so openly and frequently about using nuclear force is a clear break with history, says Frank Sauer, an international security researcher at the Bundeswehr University Munich and author of the book Atomic Anxiety: Deterrence, Taboo and the Non-Use of U.S. Nuclear Weapons. And it could be potently destabilizing in a world where nations’ nuclear doctrines are shaped more by posture than by policy.

The United States must greatly strengthen and expand its nuclear capability until such time as the world comes to its senses regarding nukes — Donald J. Trump (@realDonaldTrump) December 22, 2016

Despite a few close calls, nuclear warheads haven’t been used in armed conflict for more than 70 years. But there’s controversy over the reason why. Robert McNamara, the US secretary of defense during the Cuban Missile Crisis, put it down to pure luck.

But Nina Tannenwald, director of international relations at Brown University, argues that a taboo gradually emerged from the nuclear devastation of Hiroshima and Nagasaki. This taboo created the shared expectation that using nuclear weapons again would be deeply, morally wrong. International relations professor and author of The Tradition of Non-Use of Nuclear Weapons, T.V. Paul disagrees, arguing that it’s not a taboo but a tradition that’s driven by social and political pressures. And underlying both of these explanations is humankind’s deep-seated fear of going extinct, Sauer says.

While mutually assured destruction — the notion that any country launching nukes would likely also be destroyed by nukes — gets the most ink in terms of deterrence, these cultural and psychological deterrents play powerful roles.

Let’s be very clear: the president alone controls the nukes. There aren’t more checks and balances because our nuclear chain of command was built to speedily deliver mutually assured destruction. In fact, the only real check on the president’s nuclear authority is the election, writes nuclear history professor Alex Wellerstein in a recent blog post. “[D]on’t elect people you don’t trust with the unilateral authority to use nuclear weapons.”

That’s because if the US is attacked, time is precious: early warning teams only have three minutes to determine whether warnings of a missile attack are real. If it looks legitimate enough to take to the president, the president then has less than 12 minutes to open the nuclear briefcase (or “football”), review his tactical options, and authorize a nuclear strike. Or at least, 12 minutes is how long the White House has if a submarine deployed in the Western Atlantic were to fire on DC; if Russia were to launch a nuke from within its borders, there’s maybe 18 additional minutes to react. If the president hesitates, a nuke could hit the White House before the US has a chance to launch a counterstrike.

Still, many experts agree that mutually assured destruction can’t fully explain why no one is using their nukes. After all, the US didn’t use nuclear weapons against Iraq in the 1991 Gulf War, even though Iraq didn’t have any nuclear weapons to retaliate with.

International law isn’t a great deterrent, either. True, the United Nations Charter does ban military force except in self defense, and using nukes could possibly constitute a war crime. The Nuclear Non-Proliferation Treaty of 1968 implicitly bans the five so-called nuclear weapon states — the US, the UK, Russia, France, and China — from attacking a non-nuclear party to the treaty. But even so, in 1996, the UN’s judicial branch ruled that it’s not illegal to use nukes to ward off an existential threat. It’s just not really legal, either.

Other nations do a better job at checking their leaders’ nuclear strength. China and India both pledged to not use nuclear weapons in a first strike. (India changed their policy in 2003 to let them retaliate with nukes against a chemical or biological weapons attack.) Russia walked back their own no-first-use policy in the 1990s, and the US doesn’t have one.

Mutually assured destruction can’t fully explain why no one is using their nukes

That’s where the nuclear taboo comes in. It lumps nuclear, chemical, and biological weapons into a category of weapons of mass destruction that are unusable precisely because they’re so powerful and hard to control, says Tannenwald, author of The Nuclear Taboo: The United States and the Non-Use of Nuclear Weapons Since 1945.

The taboo stems from the wreckage of the atomic bombs the US dropped on Japan during World War II. We still don’t know how many people were killed by the first blasts, probably between 150,000 and 250,000 in total. The death toll continued to rise over the next five years to nearly 350,000 people, with many dying of cancers from the radioactive fallout.

During his recent visit to Japan, President Obama called Hiroshima “the start of our own moral awakening.” That moral awakening has kept nations like the US from using nukes even as they stockpiled them, Tannenwald argues. The taboo casts nuclear weapons as untouchable, stigmatized tools that only a barbarian would use — shaping public opinion as well as world leaders’ personal conviction. After the bombing of Nagasaki, President Harry Truman reportedly called off any more nuclear attacks, saying, “The thought of wiping out another 100,000 people was too horrible.”

“Taboo” is too strong a word to describe how we feel about using nuclear weapons, argues T.V. Paul. Taboos prohibit things like cannibalism, he says. Cannibalism is so unthinkable that most people would never even consider it, let alone plan for how and when they’d do it. But the US government does have a plan to launch its nuclear weapons. That makes it more of a tradition, Paul says, possibly along the lines of avoiding mass killings of civilians during war. And traditions are easier to break, even though doing so would damage the country’s international reputation.

“As soon as you think nuclear weapon, you’re thinking armageddon.”

Regardless of whether it’s mutually assured destruction, taboo, or tradition — each of these deterrents stems from the same underlying anxiety about using nuclear weapons, Sauer argues. “In terms of fiction, we’ve destroyed ourselves with nuclear war 1000 times over,” he says. “We’re sort of obsessed with this. Try it in your head — as soon as you think nuclear weapon, you’re thinking armageddon.” The prospect of MAD harnesses and amplifies that anxiety. But the taboo is a way to avoid the anxiety, Sauer says — like, “I don’t even want to touch these things.”

Still, cultural norms and individual psychology are flimsy barriers to using world-destroying weapons, writes Victor Gilinsky, a physicist and former head of the US Nuclear Regulatory Commission. And they could grow weaker as memories of nuclear blasts fade. “It was a much more tangible thing years ago,” says Gilinsky, who recalls diving under his desk during bomb drills at school. “When you say you’ll explode a bomb here, explode a bomb there — it’s not a game of checkers.”

And anything that depends so much one person’s judgement is also vulnerable to that person’s ego. At some level, Gilinsky argues, anyone who’s worked on or with nuclear weapons wants to see the effort pay off. When the atomic bomb exploded on Hiroshima, the Los Alamos scientists cheered, Gilinsky recounted in a 2006 speech. Not because of the fatalities, but because their work was a success. “You’ve got these people who are constantly training,” he told The Verge. “They want it to be important. And for it to be important, the possibility of nuclear war has to be important.”

That ego doesn’t just show up as professional pride, either, Gilinsky writes in a recent article. A “cult of toughness” at the top levels of the US government could also tip the balance towards using nuclear weapons when it’s necessary in order to save face.

This kind of saber rattling could drive more nations to arm themselves

This doesn’t mean that President Donald Trump will suddenly launch a nuclear warhead and unleash nuclear armageddon. After all, a US president is unlikely to violate a long-standing taboo that the US so clearly benefits from, Wellerstein told The Verge. Densely populated cities, easy-to-locate military targets, and vulnerable infrastructure makes the US an especially exposed nuclear target if nukes suddenly became acceptable weapons to use.

But the thing about Trump’s tough nuclear talk is that even if he’s bluffing, this kind of saber rattling could drive more nations to arm themselves with nuclear weapons. Even in Germany, people were unsettled when Trump talked about withdrawing military support from NATO countries. The editor of the conservative German newspaper Frankfurter Allgemeine Zeitung wrote an op-ed speculating that Germany should build its own nuclear arsenal.

If the president wants to ensure a stable and secure Europe and Asia, he’ll need to dial back the off-the-cuff nuclear remarks. “The traditions, the taboos, the deterrences ... are all about constantly reiterating and saying we’re not doing that, this is wrong, this is right,” Sauer says. “If you undercut all of this, in a couple of not-very-carefully thought through moves, you’ll do quite a lot of damage that will take awhile to repair.”

Updated January 20, 2017: Updated to reflect President Donald Trump’s inauguration, and to include his most recent statements about nuclear weapons.

Updated December 22, 2016: Updated to include President-elect Donald Trump’s and Russian President Vladimir Putin’s recent statements about nuclear weapons.

First published December 11, 2016.