With barely a single working day left until Christmas, Vladimir Putin and Donald Trump appeared to unexpectedly announce an intensified nuclear arms race.

The Obama administration has occasionally sent nuclear capable B-2 and B-52 bombers to Asia and Europe to reassure U.S. allies. Here, a B-2 Spirit returns from flying a mission over Iraq, March 27, 2003. REUTERS/Cherie A. Thurlby/U.S. Air Force

It was, perhaps, an early sign that relations between the U.S. and Russian leaders may not be as positive as some had expected.

It is still not entirely clear exactly what Trump meant with last Thursday’s tweet that the United States “must strengthen and expand its nuclear arsenal until such time as the world comes to its senses regarding nukes.” The tweet appeared to be a response to comments by Putin earlier in the day on strengthening Russia’s atomic arsenal and bragging of his country’s ability to defeat any potential adversary.

What is clear is that Trump’s words seem a significant departure from the approach of Barack Obama, who was still trying to draw down Washington’s nuclear weapons stocks.

In truth, however, the outgoing administration was itself in many ways more visibly assertive than any of its post-Cold War predecessors in its own nuclear posturing.

Faced with an increasingly aggressive Russia and belligerent North Korea in particular, the Obama administration has on several occasions made a point of sending nuclear capable B-2 and B-52 bombers to Asia and Europe to reassure allies and send a not-so-subtle message to potential nuclear-capable adversaries.

Nuclear arms negotiations have traditionally been long, drawn out, and pointy-headed affairs. The fact that Washington’s incoming president is using Twitter to signal his intentions is a new development, and not necessarily one that will make the world a more stable place.

The United States was already planning to update its Cold War era nuclear arsenal, much of which relies on outdated technology and which has suffered string of management and technical issues in recent years, some acutely embarrassing.

Simultaneously, however, the Obama administration had been signaling its intention to tweak the U.S. nuclear stance, perhaps ruling out Washington’s “first use” of nuclear weapons in any conflict. (Under previous U.S. doctrine, particularly during the Cold War, Washington always signaled it might be prepared to go nuclear first in order to hold an overwhelming conventional attack such as one by Russian forces in Europe.)

That adjustment now seems unlikely - not least because a new Trump administration might overturn it in less than a month.

Trump has sent a range of mixed - and often unconventional - signals on nuclear policy. During the election campaign, he appeared to suggest that the Washington should roll back its historic position of pledging to respond in kind for a nuclear attack on close NATO or Asian allies, even appearing to suggest countries such as Japan and South Korea might do better to acquire their own atomic arsenals.

Given widespread concerns in the U.S. security establishment over Trump’s potentially warmer approach to Putin, there will be many on both sides of the aisle who would welcome a tougher U.S. line. The risk, though, is that it reintroduces the risk of a potentially cataclysmic confrontation between two leaders who have yet to define their relationship.

Outside the box thinking on nuclear strategy is not itself a bad thing – the reliance on “mutual assured destruction” since the early Cold War has its own measure of insanity. Maintaining that awkward and dangerous stability, however, has always been dependent on all sides acting in predictable ways.

Without the threat of atomic Armageddon, the world’s superpowers might well have spent the last 75 years fighting conventional wars that could have proved almost as unpleasant.

A poll of national security experts concluded in 2015 that the risk of a major nuclear exchange had risen over the decade. On balance, they saw a 6.8 percent chance of a major conflict in the next 25 years, killing more people than World War Two’s roughly 80 million. It’s hard not to conclude that the prospect has increased since that survey.

North Korea continues its warhead and missile testing program, its clear aim to build a basic submarine-based nuclear deterrent that would allow it to strike Japan, South Korea and regional U.S. bases. Tensions with Russia and China have also ratcheted up on a wide variety of fronts, with both also modernizing their own atomic arsenals.

In an era of cyber attacks and hybrid warfare, the internationally understood rules governing such confrontations are also dangerously in flux. The very unpredictability of Trump - and his untested, undefined approach to international affairs - adds yet another wildcard.

Russia, indeed, has been relatively open about its new doctrine of the “de-escalatory nuclear strike” – the concept of using the destination of a single nuclear warhead to force the end of a limited conventional conflict. It’s uncertain, however, whether that would work: NATO doctrine makes it clear nuclear force would most likely be responded to in kind.

Ultimately, it could be argued that both Trump and Obama - and perhaps also Putin - have just been trying to publicly wrestle with a simple awkward reality: that the threat of a major nuclear exchange between major states, particularly Russia and the United States, never really went away.

According to the Arms Control Association - one of the more respected estimators of nuclear arsenals - Russia has some 7,300 warheads in total, approximately 1,700 immediately deployable via missiles and bombers. The United States has slightly fewer - some 7,000 warheads, approximately 1,300 deployed.

No one else has anything close - the United Kingdom, France and China have approximately 200 to 300 each. India and Pakistan have an estimated 110 and 140 respectively. North Korea, which everyone worries over the most, was estimated to have a mere eight.

The U.S. and Russian arsenals are much more limited than they were at the height of the Cold War. Then, the U.S. stockpile alone stood at more than 25,000. Even today, however, a Washington-Moscow confrontation could devastate the entire northern hemisphere.

As Putin made clear in his press conference last week, that is why Russia reacted so angrily to the development and deployment of relatively limited U.S. missile defenses, particularly in Eastern Europe. From Washington’s perspective, the focus of these systems was always on rogue states, the potential risk from places such as North Korea and Iran. For Russia, however, the defenses have always been seen as a Western strategy to negate Moscow’s atomic arsenal.

Such worries were not, perhaps, entirely reasonable. Even the boldest suggested expansion of U.S. missile defenses would never have been able to shoot down more than a tiny proportion of Russia’s warheads. But with stakes like that, common sense is a relative concept.

Trump may well be right about one thing. There seems little prospect of the world “coming to its senses” anytime soon. The risks of that are now coming back into plain sight in a way not seen in a generation - and it will be his job to manage them.

UPDATE: This column has been corrected to reflect the number of warheads owned by France.