Somewhere deep in a bunker, Ayatollah Ali Khamenei is confronting the most momentous decision faced by any Iranian leader since the Revolution of 1979. One phrase sums up his dilemma – must he drink the “cup of poison”?

Four decades ago, he was a loyal follower of Ayatollah Khomeini, the previous supreme leader, who vowed never to relent in Iran’s “sacred” war against Saddam Hussein’s Iraq.

For years, Khomeini adamantly rejected any thought of compromise. Then, in July 1988, he realised that Iran could take no more and, in his words, drank the “cup of poison” and allowed a truce.

“After accepting the ceasefire, he could no longer walk,” wrote Khomeini’s son, Ahmad. “He never again spoke in public.”

Eleven months later, Khomeini was dead, and his quietly dependable lieutenant became his successor. Now Khamenei is brooding over his own “cup of poison” – only this time the stakes are even higher.

With Donald Trump demanding “unconditional surrender” and threatening to hurl US military might into the Israeli campaign, Khamenei will realise the gravity of the situation.

Unlike in 1988, the very survival of the Islamic Republic hangs in the balance.

In the skies above, Israeli jets have demonstrated their ability to strike almost any target at will. Last Friday, they wiped out all the generals in the Islamic Republic’s high command. On Tuesday, they killed one of their successors.

As well as bombing the uranium enrichment plant at Natanz and the conversion facility at Isfahan – two crown jewels of Iran’s nuclear programme – Israel is destroying the regime’s most vital infrastructure, including refineries and oilfields.

If Mr Trump sends US forces into action, the military might arrayed against Iran will be many times greater. And all of this colossal firepower is designed to exert maximum pressure on one 86-year-old man, and force him to emulate his late master and do what he has always sworn never to do.

Accepting US and Israeli terms would require Khamenei to sacrifice the entirety of Iran’s nuclear programme, especially its ability to enrich uranium. Yet he has spent the past two decades vehemently refusing any compromise on exactly that issue.

Why? Because enrichment is the vital process that could be used to produce the weapons-grade uranium at the core of a nuclear bomb. Only a handful of countries have this ability.

Under Khamenei, Iran became one of them. The cost was immense, both financial and human, because Israel began assassinating Iranian nuclear scientists as long ago as 2010.

And the Ayatollah began this momentous project decades ago. In the 1990s, he started building a clandestine uranium enrichment plant at Natanz, found in the Dasht-e-Kavir, the Great Salt Desert, about 200 miles south of Tehran.

The plan was that Iranian scientists would master the whole process in total secrecy, leaving Khamenei free to decide whether to go the whole way and order them to produce weapons-grade uranium for a nuclear bomb.

But the plot failed when Natanz was discovered and its existence announced to the world in 2002. Suddenly the regime came under huge international pressure to halt its enrichment programme.

Khamenei’s response was not just to defy these demands but to construct a second secret enrichment plant, hidden in a mountain at Fordow. Once again, the scheme failed when America and Britain jointly disclosed the existence of Fordow in 2009.

Crippling economic sanctions were then imposed to compel Iran to stop enriching uranium. And still Khamenei pressed on, even as the world’s richest countries set out to cripple the Iranian economy.

His defiance was rewarded in 2015, when the US and its allies signed an agreement that allowed Iran to keep its enrichment capacity, albeit under tight restrictions and constant inspection.

This was the deal scorned by Benjamin Netanyahu, the Israeli prime minister, and destroyed by Mr Trump during his first term. Whereupon the Khamenei resumed expanding Iran’s nuclear programme, amassing about 20,000 centrifuges and over eight tonnes of enriched uranium in the teeth of threats and pressure.

Now, as bombs fall on Iran, probably destroying or disabling all the centrifuges at Natanz, every fibre of the Ayatollah’s being will urge him to stand firm and give the same answer as before – that Iran will never relinquish its hard-won ability to enrich uranium.

But one factor – and one alone – might force him to change his mind. Despite Mr Trump’s threats, it will not be personal safety. Khamenei may be an obscurantist fanatic, but he is no coward. He would gladly trade his own life for what he sees as Iran’s nuclear rights.

Yet he has one supreme and overriding responsibility, and that is to guarantee the survival of the regime. He will hope that he can stand firm against Israel and the US while still ensuring that the Islamic Republic outlives this crisis.

Hence Mr Netanyahu and Mr Trump are striving above all to convince the Iranian leader that this option does not exist.

Everything they do is designed to force him to contemplate the downfall of the regime, pushing him to the brink, and making him understand that accepting their terms and sacrificing the enrichment programme is the only way of preventing that outcome. This is the ultimate example of power politics in action.

Khamenei will probably not dwell on how his own decisions have led him to this pass. The nuclear programme, designed to make the regime impregnable, has instead brought it to the edge. The terrorist groups that Iran sponsored to deter an Israeli attack – particularly Hizbollah in Lebanon – have instead been eviscerated.

The resources squandered on that futile cause could have been used to strengthen Iran’s air defences and prepare for this onslaught. Instead, Israeli jets are striking with impunity. And all around, Israeli intelligence has obviously penetrated every nook and cranny of Khamenei’s regime.

By his own folly, he has brought this fate upon himself. Now only one question remains – will he crack and drink the poison?