Analysis: For a couple of hours last Friday the world seemed plunged into an existential danger not felt since the meltdown of the Chernobyl nuclear power plant back in the 1980s.

Ukrainian Foreign Minister Dmytro Kuleba sent out an alarming tweet saying the Zaporizhzhia nuclear power plant was under Russian attack and on fire.

In an ominous warning, he said it had the potential to be 10 times worse than the 1986 Chernobyl nuclear disaster. As it turned out, a training building was destroyed but the reactors were mercifully untouched.

The very fact Russian forces were willing to conduct a battle in and around a nuclear power plant was condemned by Western leaders.

Speaking at a hastily convened meeting of the UN Security Council, US ambassador Linda Thomas-Greenfield described the fighting as "incredibly reckless and dangerous. And it threatened the safety of civilians across Russia, Ukraine and Europe".

The situation brought into sharp focus the terrible risks attached to conducting a war around nuclear facilities. Had a reactor been hit and a Chernobyl-style meltdown occurred, the world would be in a very different situation right now.

But for all the danger and drama attached to that all-too-real risk, it's not the only nuclear danger ahead.

The nuclear effect is being felt in Ukraine

The lesson of Russia's invasion of Ukraine is the possession alone of nuclear weapons offers those countries that have them virtual immunity from an effective military response.

Despite being outgunned and outnumbered, the Ukrainian military has been praised for its courageous resistance against overwhelming odds.

But while promises of weapons and humanitarian aid from the West have been welcomed in Kyiv, what embattled Ukrainian President Volodymyr Zelensky really wants is direct military support. Boots on the ground, planes in the sky, ships on the sea.

From the beginning, the only credible military organisation big enough to actually help, NATO, has said that sort of military intervention is not going to happen.

The Ukrainian president shifted focus, demanding NATO do what it has done in other conflicts and impose a no-fly zone over Ukraine. The answer was again, sorry, no.

A bitter Zelensky responded: "Today the alliance's leadership gave a green light to the further bombardment of Ukrainian towns and villages, refusing to establish a no-fly zone."

But NATO had already made it clear it would not risk direct conflict with Russia unless one of its member states came under attack.

A no-fly zone, if properly enforced, would inevitably lead to direct confrontation with Russia.

US Secretary of State Anthony Blinken put it bluntly, saying a no-fly zone was not going to happen as "that could lead to a full-fledged war in Europe".

At its core, the fear is that a full-blown conflict could trigger a global nuclear war.

Take nuclear weapons out of the equation and the West may have been tempted to put troops and tanks into Ukraine. But the threat of nuclear conflagration is seen as too great a risk to take. And President Vladimir Putin knows it.

Several times in the past weeks he has reminded the West he is not afraid to use nuclear weapons to defend Russia. Despite the imposition of sanctions and promises of "lethal aid" to help the Ukrainians fight the Russians on the ground, they are on their own.

When Ukraine became an independent nation, with the dissolution of the Soviet Union, it gave up its sizeable nuclear arsenal in return for security guarantees offered by the United States and Russia.

If it had kept its nuclear missiles it is possible to argue Ukraine may not be facing this Russian invasion. And that is because Putin would be the one calculating the risk, and perhaps deciding it wasn't worth it.

The lesson for countries around the world is that the mere possession of nuclear weapons works.

More nations are going nuclear

Despite global efforts to contain the spread of a fearsome threat, the list of nuclear states has expanded from the original five: Russia, the US, France, Britain and China.

Both India and Pakistan have joined the club, as have North Korea and Israel, which has never officially confirmed its nuclear capability.

No-one has leveraged that new status as effectively as the Kim family dynasty in Pyongyang.

In any other circumstance, the power and prestige of a US president would never be compromised by agreeing to a summit with a small-time despot like Kim Jong Un.

But North Korea has nuclear weapons and intercontinental ballistic missiles that could reach the west coast of the United States or Australia. That fact alone put Kim at the same table as former US president Donald Trump at summits in Singapore and Hanoi.

It was an extraordinary sight, with the supposed leader of the free world exchanging pleasantries with a dictator with one of the worst human rights records on the planet. And all because he had the bomb.

Far from discouraging states from attempting to acquire nuclear weapons, the sad truth is recent history proves ownership of the means to destroy entire cities, if not countries, really is a protection against meaningful opposition.

Vladimir Putin is using the fear of Armageddon to devastating effect. There will be no NATO cavalry riding in to save the day in Kyiv.

For all the powerful words of support for the Ukrainians from around the world, they will carry the fight, and the suffering, alone.