When Ashton Carter began his career at the Pentagon, in 1993, geopolitics was changing more rapidly than it had at any point since the Second World War. As the Cold War ended, a new world was taking its place, one dominated by American power.

Carter, at the time, expected that America's greatest challenge in that world — and, by extension, the focus of his own career — would be limiting the spread of nuclear weapons, he told me in a recent interview at the Pentagon. And the greatest threat, he thought, would be political instability within nuclear-armed Russia.

"That was the riveting challenge of that time. We look back now, and I remember how fearful that was," he said. "But it worked out extraordinarily well."

Twenty-some years later, Carter now runs the Pentagon as defense secretary. The world, and the challenges it presents for the United States, turned out somewhat differently than he, or anyone else, had expected.

What Carter has seen over the past two decades, and has often overseen, is the long and difficult process whereby the United States has tried, and at times struggled, to navigate those unexpected turns in the grand experiment that is the post–Cold War world.

The challenges of this era have only recently become clear: the failure and collapse of weak states, which can bring terrorism, civil war, and refugee influxes; rogue states that resist the American-led order and proliferate dangerous weapons; and now, according to Carter, a return to "great power competition."

In a lengthy and wide-ranging conversation, Carter expounded at length on those challenges: how they've changed, what the US has learned in its successes and stumbles with them, and what he sees coming. You can read the interview in full here.

But I was struck, in our conversation, by the frequency with which Carter framed today's world within those same dynamics that had preoccupied his early career: great power rivalries, nuclear weapons, and the power of deterrence to keep the world in line.

It's not that he's unconcerned with terrorism or rogue states, both of which he discussed at length. But he clearly drew from his experiences in the 1980s and '90s an acute sensitivity to the world-shaking stakes of great power rivalries, and a firm belief in the role that deterrence and nuclear weapons still play.

Carter described a world where not just American power but also global peace and stability will be increasingly challenged by other powers. He returned over and over to the same answer to this problem: the power of deterrence, backed up by overwhelming American superiority and, ultimately, by nuclear weapons.

But deterrence works on decades-old understandings of warfare that are, today, rapidly changing. As countries develop new tools and techniques of war, Carter warned that deterrence, the foundation on which American power and global stability are built, needs to change as well.

That is not a challenge that grabs as many headlines as terrorism or rogue states. But the stakes — the international order as know it, 70 years of peace between the major powers, and the thousands of nuclear warheads backing it up — could not be higher.

A changing world

In an institution as vast and expensive as the American military, budget and strategy can often be synonyms. And Carter, in writing and selling the final Pentagon budget of the Obama era, has said he is trying to prepare the United States for "a return to great power competition."

That is a world, he has said, in which Russia and China, while still not America's military equals, can nonetheless challenge the American-led post–Cold War order. It's a problem "we haven't had to worry about ... for 25 years" but that is becoming increasingly real in Europe and in the Pacific.

I asked Carter why he believed this was happening — what is driving the problem, and thus, by extension, what is the appropriate response?

In answering, he characterized Russia and China as driven not by cold, hard power politics — which would thus suggest the US could negotiate a mutually beneficial resolution to any disagreement — but rather by the ideologies of their governments.

Vladimir Putin's Russia, he suggested, is "trying to justify itself to its people on how much it can stand against the West." This has led it to "emphasize military confrontation with the West, anti-Western propaganda, and especially the nuclear dimension of that."

As for China, he described that country's recent military expansionism as less cynical than Russia's, instead rooted in historical grievances and suspicion of American power.

While Carter insisted the US has no problem with China's power growing commensurate to its economy, he suggested that the country is also driven by "the idea [in China] that we need to right the wrongs of the past and dominate our region, and reject the system of rules-based order that we associate with the United States."

While the American-led order serves China, he argued, "There's a part of the Chinese mind that thinks that's an American creation, rather than a good in itself."

"In China, it's a feeling of destiny about dominating a region rather than participating in a region," he went on.

While Carter was careful not to call the Chinese leadership irrational, he clearly sees Beijing as acting against its own interests. He had hoped, he said, that "the logic of the situation would ultimately prevail over the emotion of history." Until it does, "we have to understand that China is building up its military, it's trying to intimidate many of its neighbors."

In response to Russian and Chinese actions, he said, "I am making investments in high-end capabilities of a kind we might not have thought 10 years ago we'd need to make. We're trying to catch up in some areas, advanced technology areas, with respect to China."

This is a somewhat different attitude from what you hear in, say, the State Department, where many policymakers share Carter's criticisms of Beijing and Moscow but believe the US should respond with greater engagement, using sticks and carrots to encourage cooperation and discourage what they see as bad behavior.

Carter seems to take a harder line, seeking primarily to deter Russia and China rather than negotiate with them.

"Negotiating with other great powers about how we can together help the world progress to mutual benefit, all of that is fine," he said. "But if you have the attitude that you're aggrieved, and pressing your grievance rather than negotiating the future is what it's about, that makes it very difficult for us."

I suspect this is less a function of Carter's personal views than it is of the Pentagon's traditional role in US foreign policy, which has been one of upholding and defending the international order against other powers — an approach that is more zero-sum.

There is a long-held worldview in the US military that sees American military dominance as providing stability, and this stability as enabling much of the peace and prosperity of the postwar era. In that view, any challenge to American dominance is a challenge to global peace and prosperity itself.

"What's kept the peace in that region all those many decades," Carter said of Asia-Pacific, "was the system of rules-based order and the pivotal role of the American military in the region."

Carter also seems to see renewed great power competition as a dynamic that China and Russia have forced onto the US — meaning, in his view, that it is on them to end it.

When I pointed out that Moscow and Beijing see themselves as merely seeking a larger role in the world commensurate to their growing power, he responded, "In Vladimir Putin's Russia, you have a deliberate tinge of thwarting America as an end in itself, not a, 'Hey, look, we need to sit down together and accommodate one another.'"

Unlike Secretary of State John Kerry, who often meets and appears publicly with Russian Foreign Minister Sergei Lavrov, it is difficult to imagine Carter doing lots of glad-handing with his Russian counterpart.

The fog of hybrid and cyber war

If the problem is renewed great power competition, then Carter's solution is an updated version of the strategy that saw the US through the Cold War: deterrence.

Under deterrence theory, countries give one another clear warnings about what actions will trigger a retaliation. And they make sure those warnings are backed up by overwhelming military force, such that no country would dare cross another's red lines.

A classic example is Cold War–era Berlin: The US and its allies made clear that if the Soviet Union invaded West Berlin, they would declare war. In this way, countries can protect their vital interests — in this case, a free West Berlin — while also preventing the outbreak of war.

Deterrence is why the Cold War stayed cold, and it's why the world's major powers have not fought a war with one another since 1945. It still applies today, but as the technology and norms of warfare are changing, Carter says deterrence needs to change as well.

"As we get into new domains, like cyber and space, and we try to situate old and very solid ideas like deterrence in those new domains, it requires some thought," he said. "In the case of space, for example, we're still developing those understandings."

This was an idea he mentioned several times in our conversation: updating deterrence to apply to a changing world. And one of the most urgent changes has been Russia's development of new, asymmetric techniques sometimes called hybrid war.

"We're reinvigorating NATO with a new playbook, different from [that of] the Cold War, to deal with, for example, the little green men phenomena that you saw in Ukraine," Carter said, referring to the unmarked Russian special forces that seized Crimea in 2014.

Deterrence, after all, is an idea from a different era, when "war" meant tanks rolling across borders, armies invading cities. And deterrence works on certainty: Certainty about who is doing what, and what actions will spark retaliation.

Hybrid war is all about creating uncertainty: special forces dressed up as vigilantes, attacks meant to create chaos more than seize territory, propaganda and misinformation meant to spark panic or popular unrest.

These sorts of attacks are deniable — how do you threaten to retaliate against an attack if you can't prove who launched it? And they exist outside the norms of what we do and do not consider war, potentially allowing states like Russia to subvert norms of deterrence.

When I asked Carter about this, he answered that deterrence had to be about more than just coldly calculated red lines and retaliatory threats. It also needs to have a kind of psychological effect, and thereby compensate for any uncertainty created by line-blurring methods such as hybrid war.

"It has the word 'terror' inside it, right? 'Deterrence,'" he said. "That's what it's about, is you scare someone away from doing something you don't want them to do by making them fear the consequences."

In other words, even if countries like Russia can use new tools such as hybrid warfare to blur or inch across the traditional lines of deterrence, then the US should make sure that the stakes of getting caught are sufficiently high to deter them from doing so.

"If you're attacking American interests, there will be a response," he elaborated later in the conversation. "It's important to communicate that fact, that you're prepared for it, we know exactly what we're going to do, and you will regret the consequences of your act and you should be fearful of those consequences and thus not do it in the first place."

This problem is especially acute with regards to cyber war. As defense analysts increasingly warn, the world has not yet established clear norms for cyber conflict.

When does a state-sponsored hack become an act of war? Is it when the hackers steal military secrets? When they attack military communications? What about if they cause physical damage, as Iranian state-sponsored hackers could have done, but did not, when they accessed the controls for a dam in New York state?

The world has not yet established clear, agreed-upon answers to these questions. This makes it harder for the US to effectively deter cyber attacks because it has trouble articulating what will draw a response, and what sort of response.

Carter acknowledged that these norms are still being established. But he again argued that overwhelming US military superiority would deter even the sorts of attacks, such as cyber, to which deterrence has not traditionally applied.

"If you attack us in cyberspace, it's an attack," he said, as if speaking to a hypothetical adversary. "I'm very straightforward. An attack is an attack. We won't necessarily respond in space, or respond in cyber, we may respond in some other way, but we will respond and you need to understand that."

Carter also nodded to a risk that many in NATO have been warning about since 2014: that some Russian provocation, calculated to blur lines of deterrence without crossing them, could overreach, unintentionally crossing those lines by accident or miscalculation, blundering Europe into an unwanted conflict.

"You can't stop a conflict that your enemy deliberately provokes, but you can try to prevent ones they blunder into by underestimating you," he said. "One of the ways you do that is by signaling clearly and having dialogue."

The way to reduce this risk, in his view, is for the US military to regularly communicate with the Russian and Chinese militaries, so as to prevent any misperception and control any incident from spiraling out of control.

But when I asked Carter if he was satisfied with the level of military-to-military cooperation, he answered flatly, "No."

"I'd like to see more in general, but it takes two to tango," he said. "It's not just the amount; it's the character of the dialogue and the willingness of those parties to have a dialogue of a kind that we would regard as fruitful. That's not everything that it should be."

"The single most fearsome and dangerous technology created by humankind"

Any conversation about deterrence or modern great power rivalry will inevitably turn to the piece of technology that has been at the heart of major power dynamics since the 1950s: nuclear weapons.

Their terrible power, after all, is what makes deterrence so effective — who would risk a war that could literally end the world? — but also so dangerous.

To make the nuclear deterrent credible, we have dotted the globe with air-, land-, and sea-based warheads, ready within a few minutes' notice to target the world's major cities — a threat to kill millions made credible enough that, we hope, it never has to be carried out.

Carter, who has worked on nuclear weapons for decades, is intimately familiar with their power and with the intricate theories that have led us to build 7,100 warheads and deploy more than 1,500.

But I asked him to consider a story of someone confronting the logic of nuclear deterrence for the first time.

In 1989, President George H.W. Bush appointed a Wyoming Congress member named Dick Cheney as secretary of defense. New to the role, Cheney traveled to Nebraska, where US Strategic Command is based, to familiarize himself with the body that oversees, among other things, America's nuclear weapons.

Amid what were surely several long days of dry policy briefings, Cheney was shown a video on an esoteric technical issue that nuclear policy wonks call fratricide.

In a nuclear launch, you have to time your strikes carefully. Otherwise, as nuclear expert Jeffrey Lewis wrote in a column recounting the incident, "the blast, electromagnetic pulse, and debris from the first wave of nuclear weapons in an attack will damage subsequent waves."

Whoever was briefing Cheney decided the best way to explain fratricide was by showing him the US nuclear strike plans for part of Moscow. The video illustrated each strike with a red dot. As the scene stretched on, one dot after another appeared, blanketing one of the world's most populous cities. It indicated 69 nuclear strikes on the suburb of Pushkino alone.

As Cheney saw America's nuclear war plans unfold before his eyes, he reacted the way that anybody would: with horror.

"Moscow turned slowly into a solid red, covered over and over with ludicrous targets," a participant at the meeting recounted to the scholar Janne Nolan. "Cheney started squirming around and finally asked one of his military aides why we were doing this kind of thing."

Surely Carter had seen people express this reaction many times in his career, I pointed out.

"I do, I hear it all the time," he answered. "You never get quite used to how terrible such a situation would be."

I expected Carter to downplay the threat of nuclear weapons, and not just because he has been around them for so long.

Carter is supporting the Obama administration's plan to modernize the nuclear arsenal, expected to cost up to $1 trillion over the next 30 years. In my experience, when policymakers want to spend a trillion dollars on something, they tend to gloss over any risks that something might pose, such as, in this case, global annihilation.

But while Carter emphasized the physical security of America's nuclear weapons, he did not downplay their threat.

"I don't think that any person who understands nuclear weapons could ever change his or her thinking in this regard: They are the single most fearsome and dangerous technology created by humankind," he said.

So why have them? Carter's answer, again, was deterrence. Only nuclear weapons can deter nuclear weapons.

"Until someone has an alternative to deterrence as dealing with someone who might use nuclear weapons against you — and we haven't found, in all those years since the Manhattan Project, any effective defense against nuclear weapons — until those are found, the only defense we have is the threat of retaliation," he said.

Later in our conversation, he said of nuclear weapons, "The point of having it is to reduce the risk that anybody uses nuclear weapons against us. In that sense, its whole purpose is to reduce risk."

But there are moments when the risk associated with nuclear weapons increases, even if only incrementally. We are in one such moment now. Partly that is because of the softening lines of deterrence, undermined by new technology and techniques such as cyber and hybrid war.

But it is also because of something particular to nuclear weapons: The US and Russia, which together possess about 90 percent of the world's warheads, are both upgrading their nuclear deterrents. But nuclear weapons are simultaneously defensive and offensive. So when you strengthen your ability to deliver those weapons, you are increasing the danger you pose to the other side. And that is destabilizing.

The US, for example, is developing a new air-launched cruise missile that can deliver nuclear strikes more quickly and stealthily. From one perspective, the US is merely keeping pace with Russia's improving missile defense systems, preserving the status quo equilibrium.

But from another, the cruise missile forces Russia to reduce its response time to any perceived attack and to treat any conventional cruise missile launch as a potentially nuclear strike, thus reducing the world's margin for error and increasing the risk of "triggering a nuclear war at a time of tension," as British Foreign Secretary Philip Hammond put it.

I asked Carter, whether these sorts of risks are just the cost of maintaining nuclear deterrence.

"Our focus is really on maintaining the nuclear deterrent that we have. We're not looking to increase its size. We're not looking to do anything novel or different with it," he answered.

But he went on, "The Russians are also very rapidly modernizing their own nuclear arsenal. I don't associate that with what we're doing. I associate it with the dynamics of their own feelings that nuclear weapons are one of the only things that guarantee their status in the world."

"They're fueling their own nuclear modernization," he stressed. "It's a mistake to think that we're fueling it."

Maybe so, but the United States is still upgrading its nuclear capabilities at a time of rising tension and blurring lines of deterrence between nuclear powers, and with new hardware that even former Defense Secretary William Perry has called "destabilizing."

Maybe this is simply necessary for maintaining deterrence. But that deterrence does carry risks. And those risks, in an extremely unlikely but not impossible worst-case scenario, include nuclear conflict. These weapons exist to deter, but that only works if the threat to use them is credible.

As Carter put it, deterrence has the word "terror" in it for a reason.