Just days before his inauguration, Donald Trump made headlines by trashing America’s European allies in an interview with two of Europe’s biggest newspapers. The hubbub over Trump’s attack on Europe obscured one of the stranger comments in the interview — that he hoped to work with Russian strongman Vladimir Putin to reduce both countries’ nuclear arsenals.

“Let’s see if we can make some good deals with Russia,” Trump said. “For one thing, I think nuclear weapons should be way down and reduced very substantially.”

To say this is a flip-flop is an understatement. Less than a month ago, Trump tweeted that the US “must greatly strengthen and expand its nuclear capability until such time as the world comes to its senses regarding nukes.” When MSNBC’s Mika Brzezinski asked him about the possibility of this policy setting off an arms race with Russia (which is also talking about expanding/modernizing its nuclear arsenal), Trump’s answer was simple.

“Let it be an arms race.”

Nuclear arms control is a hugely important issue — especially in a world where tensions between the US and two other nuclear powers, Russia and China, are heating up. So where does Trump stand: with his comments from December, or with his comments from January?

The truth is that nobody knows — leaving us in the dark on one of the very few policy issues with the potential to transform the future of human civilization.

“It's difficult to discern what Trump's policy will be and whether he has given more than a few minutes’ thought to these issues,” Kingston Reif, the director for disarmament and threat reduction policy at the Arms Control Association, tells me.

The strange history of Trump on nuclear weapons

Questions about nuclear policy have dogged Trump for more than a year now. In a December 2015 Republican debate, moderator Hugh Hewitt asked Trump about the “nuclear triad” — America’s three-part system for delivering nuclear weapons (bombers, submarines, and intercontinental ballistic missiles). Trump’s answer was confusing:

We have to be extremely vigilant and extremely careful when it comes to nuclear. Nuclear changes the whole ballgame. ... The biggest problem we have is nuclear — nuclear proliferation and having some maniac, having some madman go out and get a nuclear weapon. That's in my opinion, that is the single biggest problem that our country faces right now.

There are two interesting things about this. First, Trump suggests, as he later did in his January interview, that he sees large global nuclear stockpiles as a problem. Second, he doesn't appear to know any of the major policy questions surrounding the nuclear triad, or even what the nuclear triad is.

That became especially clear when Hewitt followed up, pressing Trump to answer the actual question about the triad. Trump’s response? “I think — I think, for me, nuclear is just the power, the devastation is very important to me."

This pattern — an abstract abhorrence of nuclear weapons but seemingly confused views on actual nuclear policy — continued throughout the campaign. In a March 2016 town hall, for example, host Chris Matthews pressed Trump on whether he’d use nuclear weapons. He seemed to say both no and yes at the same time, saying he’d be “the last one to use nuclear weapons,” but also that he would be very willing to nuke ISIS territory in response to a terrorist attack:

TRUMP: I’d be the last one to use the nuclear weapons, because that’s sort of like the end of the ballgame. MATTHEWS: So, can you take it off the table now? Can you tell the Middle East we are not using the nuclear weapon on anybody? TRUMP: I would never say that. I would never take any of my cards off the table... MATTHEWS: Where would we drop — where would we drop a nuclear weapon in the Middle East? TRUMP: Let me explain. Let me explain. Somebody hits us within ISIS — you wouldn’t fight back with a nuke?

In August, MSNBC host Joe Scarborough recounted a story an unnamed foreign policy expert told him about Trump and nukes. In it, Trump expresses confusion as to why the US doesn’t use its nukes.

"Several months ago, a foreign policy expert on the international level went to advise Donald Trump. And three times [Trump] asked about the use of nuclear weapons. Three times he asked at one point if we had them why can't we use them," Scarborough said.

During the first general election debate, in September 2016, moderator Lester Holt asked a more specific version of Matthews’s question — what Trump thought about a “no first use” policy. That’s the idea that the US should swear off launching a nuclear strike against an enemy unless it has been attacked with nukes first.

Here was Trump’s answer:

Russia has been expanding their — they have a much newer capability than we do. We have not been updating from the new standpoint. I looked the other night. I was seeing B-52s, they're old enough that your father, your grandfather could be flying them. We are not — we are not keeping up with other countries. I would like everybody to end it, just get rid of it. But I would certainly not do first strike. I think that once the nuclear alternative happens, it's over. At the same time, we have to be prepared. I can't take anything off the table.

Trump’s answer, once again, gestures at hating nuclear weapons (“just get rid of it”). But he also implies that he supports developing more advanced ways of delivering nuclear weapons — which the US is already doing.

But when it came to Holt’s actual question, about his views on no first use as a policy, Trump had no real answer. He’s said both, “I would certainly not do first strike,” and, “I can’t take anything off the table” — but those are opposite things. The whole point of a no-first-strike policy is taking a first strike off the table. That means Trump literally had never heard of the no-first-use debate, had never thought about it enough to have an actual opinion, or didn't for some reason want to say that that opinion was.

By the time Trump won the election, experts on nuclear policy were thoroughly confused. There was simply no consistent policy throughline, no meaningful ability to figure out what Trump truly plans to do with the only weapons that have the power to destroy the Earth in minutes.

“To journalists asking me what nuclear policies Trump will adopt: I have absolutely no idea,” James Acton, the director of the Carnegie Endowment for International Peace’s Nuclear Policy Program, wrote two days after Trump’s upset win. “And neither, I strongly suspect, does he.”

Trump’s actual nuclear policy has not gotten clearer

If you squint at this history of Trump comments, you can kind of put together a consistent line.

It seems that Trump thinks nuclear weapons are extremely dangerous and that the world should work on eliminating them. But in the absence of any agreements to do so, the United States should maintain and even expand its nuclear arsenal to make sure it’s deadlier than that of any peer competitor.

Read in this light, there’s some consistency between Trump’s December and January lines. In December, he was expressing what the US should do absent arms control agreement — expand and upgrade its nuclear stockpile. In January, by contrast, he was expressing his support for an agreement between the US and Russia that might make this unnecessary.

These views have some foundation in real policy problems. As my colleague Yochi Dreazen explains, there’s a widespread worry in the national security community that America’s system for delivering nukes is starting to show cracks — to take one high-profile example, an investigation found that missileers at one base in Montana were routinely cheating on their proficiency exams, meaning they might not actually be able to launch. Failures like this, and the need to make sure nukes actually function, has led the Obama administration to devote about $1 trillion over the next 30 years to nuclear maintenance and growth.

You can support modernizing America’s remaining nukes and, at the same time, work to reduce the overall number of nukes the United States has. This is what the Obama administration has done, concluding a treaty with Russia — called New START — to cut the overall number of weapons possessed by both countries. Trump pursuing this would be welcome news to the nuclear policy community — current stockpiles are easily large enough to extinguish all life on Earth. If Trump did strike a bargain with Putin that builds on New START, that would count as a real accomplishment for his unprecedented policy of buddying up to the Kremlin.

But the issue is that it is very hard to figure out what Trump’s inchoate feelings mean in terms of actual policies like these. His comments on nukes are so vague, and so abstract, that it’s very difficult to understand how they translate in concrete policy terms.

Does Trump now support a no-first-use doctrine? Does he support entirely eliminating nuclear weapons, a goal known as “global zero,” or merely further reductions in the US and Russian stockpiles? Would he keep New START in place? What about the Anti-Ballistic Missile Treaty or the Comprehensive Test Ban Treaty, two major nuclear policy agreements?

On these points — again, the stuff that actually matters — Trump is radically unclear.

“Until we can see what exactly is on the table and what the possible outcomes might be, we will withhold judgment on Mr. Trump’s proposal,” Reif says. “The devil is always in the details.”

You can see this policy incoherence on a closer examination of the December and January comments. Remember, Trump didn’t just say in December that he’d be fine with the US modernizing its nukes — he said he’d be fine with an out-and-out arms race. If nuclear stockpiles are so dangerous, as Trump has suggested he believes, then it seems like he’d want to avoid something that would lead to their rapid growth.

But he doesn’t, really. Trump has vague, abstract feelings about nuclear weapons, but nothing in his public comments suggests he has any real sense of how those feelings translate into actual policy.

“This week it's reductions; last week it's an arms race,” Acton tweeted. “Don't treat these utterances as serious policy statements.”

Other policy experts agree.

“Unhappily, there's not much substance here on arms reductions,” Joshua Pollack, a senior research associate at Middlebury’s James Martin Center for Nonproliferation Studies, wrote earlier this week.

“Sanctions should persuade behavior shift on Ukraine, Syria & hacking. This [nuclear negotiation proposal] seems like desperate pretense for lifting,” Ben Loehrke, the policy program officer for nuclear security at the Stanley Foundation, tweeted around the same time.

Now, Trump’s national security team has at least one member who knows about nukes. Defense secretary nominee James Mattis, a retired Marine general, was asked repeatedly about nuclear weapons. Unsurprisingly, he demonstrated real familiarity with nuclear policy issues — making clear that he sees nukes as a deterrent and is deeply hostile to their use, implying opposition to the creation of a drone that could drop nuclear bombs, supporting modernizing all three legs of the triad, and the like.

“We must continue with current nuclear modernization plans for all three legs of the triad, and for associated command and control systems,” Mattis said. “What we’re trying to do is set such a stance with our triad that these weapons must never be used.”

But Mattis isn’t actually going to be in charge of nuclear weapons. That’s under the Department of Energy’s purview, a division of labor that has its origins in Harry Truman’s desire to keep the nuclear stockpile under civilian control. Trump’s nominee to lead the Department of Energy, former Texas Gov. Rick Perry, has demonstrated no meaningful knowledge of nuclear weapons. He once supported abolishing the Energy Department entirely.

Ultimately, then, we’re in the dark about how the Trump administration will handle nukes — no closer to understanding Trump’s nuclear policy than we were the day he was elected.

Watch: A brief history of America's nuclear mishaps