Russian President Vladimir Putin’s invasion of Ukraine has generated significant debate about deterrence, focused principally on Ukraine’s non-membership of NATO and the extent to which its membership aspirations represent a legitimate security concern to Russia.

But another salient detail has not escaped attention. In 1994, Ukraine agreed to destroy the nuclear stockpile—the world’s third largest—it inherited from the dissolution of the Soviet Union, in exchange for security guarantees from Russia, the United States and the United Kingdom. The aim here is not to relitigate that decision; the point is that, deprived of the two gold standards of deterrence (its own nuclear weapons or a NATO membership card), Ukraine was invaded. That won’t be lost on the handful of other medium-sized states trying to balance strategic interests in the shadow of menacing neighbours. In light of Putin’s invasion, will more states seek the ultimate deterrent?

To begin, let us define our terms. I have in mind states that satisfy the following criteria:

• They don’t already possess nuclear weapons.
• They are constrained by a large state that they perceive as a threat or might come to perceive as a threat.
• They are large enough that their acquisition of nuclear weapons is plausible.
• Their constitution doesn’t explicitly forbid the acquisition of nuclear weapons.
• They seek an independent foreign policy or are showing signs of seeking such a policy.
• They have no obviously superior means of outsourcing nuclear deterrence (that is, NATO membership).

Run the algorithm and it generates the following: Iran, Taiwan, Finland and Vietnam. Include states that satisfy some but not all of the criteria and the group extends to states such as Indonesia, Mexico, Japan, South Korea, Saudi Arabia, Serbia, Bangladesh and the two largest Central Asian states—Kazakhstan and Uzbekistan.

The four members of the first group all warrant closer analysis.

Let’s begin with Iran. As I have written elsewhere ^[1], Iran and the US are edging closer to reviving the Joint Comprehensive Plan of Action which was effectively torn up by Donald Trump. But who’s to say that Supreme Leader Ali Khamenei and President Ebrahim Raisi won’t be reconsidering those moves in light of the invasion of Ukraine? The war in Ukraine is likely to further destabilise the Middle East in two ways: by putting economic pressure on Arab leaders who are dependent on Russian and Ukrainian wheat to feed their people, and by expanding the economic power of Iran’s Gulf rivals as the West seeks to wean itself from Russian fossil fuels.

Both of these factors, together with the harsh lesson being dished out to Russia that participation in the global financial system creates significant interdependency risks, may convince Iran’s rulers that the leverage and deterrence that the bomb brings are worth whatever costs further sanctions might bring.

In a class of special cases, Taiwan is a particularly special one. There’s no evidence that it possesses or seeks to possess nuclear weapons. There has been virtually no talk of Taiwan developing domestic nuclear capabilities since the Taiwan Strait crisis of the mid-1990s. But, as its prowess in chip manufacturing demonstrates, Taiwan is highly technologically sophisticated. There’s little doubt that it could produce nuclear weapons if it wished. And, as China continues to stir up nationalist zeal and bolster its military capabilities while the US turns its gaze inward, Taiwan’s leaders may conclude that the deterrent value of strategic ambiguity has declined to the point where it should pursue its own path towards nuclearisation.

As for Finland, the defensive wars it fought against the Soviet Union have calcified into a wariness of Russian intentions. It is not a member of NATO or the EU, nor is it a party to the Treaty on the Prohibition of Nuclear Weapons ^[2]. True, Putin’s invasion of Ukraine has pushed public support ^[3] for NATO membership to record highs. But that support may subside. Finns cherish their independence. They may yet conclude that acquiring nuclear weapons is the sole means of preserving it.

Vietnam is a slightly speculative inclusion. Indeed, it is today party to most relevant non-proliferation treaties and agreements, including the Treaty of Bangkok ^[4]. But it is also poised to be the first Southeast Asian state to generate its own nuclear energy ^[5]—thanks in part to Russian assistance. It fought wars against China and the US within living memory, shares a 1,300-kilometre border with the former, and abstained ^[6] from the UN General Assembly resolution condemning Russia’s invasion. Hanoi may decide that, despite its treaty commitments, the only way to truly guarantee its security is with its own nuclear weapons.

What about Australia? As recently as the late 1960s, Prime Minister John Gorton wanted Australia to develop its own nuclear weapons ^[7]. Despite Australia signing the Nuclear Non-Proliferation Treaty in 1970 and being under the implied protection of the US nuclear umbrella, today there’s occasional debate about Australia acquiring its own weapons (strong recent pieces include one in the Australian Financial Review ^[8] and another in The Strategist ^[9]).

Certainly, China’s aggression is driving strategic competition in the Indo-Pacific to new levels. But I’m not convinced that Canberra’s calculations have been altered substantially by Putin’s invasion of Ukraine. First, it’s clear that the acquisition of nuclear submarines as part of the AUKUS agreement is partly intended to enhance Australia’s powers of deterrence. Second, Australia’s leaders are likely to conclude that there’s too much uncertainty about how Putin’s invasion will influence China’s actions in the region. While it’s true that the government has sought to make national security an election issue, there is no serious talk of developing our own nuclear weapons.

The countries mentioned here are likely to seek their security by other, non-nuclear means. But accumulate enough tiny probabilities and you will be confronted by an event with a low to medium probability of occurring. Leaders around the world may well have drawn the lesson that the seeds of Russia’s invasion of Ukraine were sown the day Kyiv willingly ceded its nuclear weapons. After all, only the most ardent hawk today seriously contemplates an invasion of North Korea.

Even a 5% probability that a new member will join the nuclear club—or even signal a desire to do so—should alter strategic calculations in Washington, Beijing, Moscow, Brussels and Canberra. When the stakes are so high, even a small possibility of a more multipolar nuclear order is worth taking seriously.