The decision by the United States and Israel to plunge into a new war with Iran creates a highly dangerous moment with unpredictable consequences. Israel used the word “pre-emptive” to justify its attack.

Israel and the United States have calculated that the Islamic regime in Iran is vulnerable; dealing with a severe economic crisis, the fallout from the brutal crackdown on protesters at the start of the year and with defences still badly damaged by last summer’s war. Their conclusion seems to have been that this was an opportunity that should not be squandered.

In their statements, both President Donald Trump and Prime Minister Benjamin Netanyahu said that Iran was a danger to their countries – Trump said it was a global danger. The Islamic regime is certainly their bitter enemy. But it is hard to see how the legal justification of self-defence applies given the huge disparity of power between the US and Israel on one side and Iran on the other.

Benjamin Netanyahu has seen Iran as Israel’s most dangerous enemy for decades. For him, this is a chance to do as much damage as possible to the regime in Tehran and to Iran’s military capacity. Netanyahu also faces a general election later in the year. The evidence from the two years of war with Hamas is that he believes his political position strengthens when Israel is at war.

Donald Trump’s objectives have veered and changed, characteristically. Back in January, he told protesters in Iran that help was on its way. Much of the US Navy was busy removing the leader of Venezuela at the time, so he lacked military options.

While the US was deploying two carrier strike groups to the region, as well as considerable land-based firepower, Trump talked a lot about the dangers of Iran’s nuclear ambitions, even though after last summer’s war, Trump declared that the Iranian nuclear programme had been “obliterated”.

The Iranian regime has always denied that it wants a nuclear weapon, but it has enriched uranium to a level that has no civilian use in a nuclear power programme. At the very least, it seems to want the option of building a bomb. So far Israel and the US have published no evidence that it was about to happen.

In his video, Trump told Iranian people that “the hour of freedom” was at hand. Netanyahu had a similar message, that the war will present the people of Iran with the chance to overthrow the regime. That is not at all certain.

There is no precedent for regime change happening just because of air strikes. Saddam Hussein of Iraq was overthrown in 2003 by a huge US-led invasion force. Muammar Gaddafi of Libya was overthrown in 2011 by rebel forces that were provided with an air force by Nato and some Arab states. In both cases the result was the collapse of the state, civil war and thousands of killings. Libya is still a failed state. Iraq is still dealing with consequences of the invasion and the bloodletting that followed.

Even if this becomes the first case of air power alone collapsing a regime, the Islamic regime will not be replaced by a liberal democracy that upholds human rights. There is no credible alternative government in exile waiting in the wings.

Over almost half a century the Iranian regime has created a complex political system that is underpinned by a mix of ideology, corruption and when required, the ruthless use of force. The Tehran regime demonstrated in January that it was prepared to kill protestors. It has security forces that obey orders to shoot and kill thousands of fellow citizens for challenging the system on the streets and demanding freedom.

Perhaps the US and Israel are trying to kill the supreme leader, Ayatollah Ali Khamenei. Israel believes in the power of assassination as a strategy. In the last two years it killed the leaders of Hamas in Gaza and Hezbollah in Lebanon, and many of their lieutenants.

The Islamic regime in Iran is a different matter. It presides over a state, not an armed movement. It not a one-man show. If the supreme leader was killed, he would be replaced, most likely by another cleric supported by the Iran’s Islamic Revolutionary Guards Corps (IRGC), which exists alongside the conventional armed forces with the explicit task of defending the regime against threats at home and abroad.

Trump offered them immunity if they laid down their arms or certain death. The IRGC is unlikely to be tempted by his offer. Martyrdom is a constant motif in the ideology of the Islamic Republic and in Shia Islam.

Trump believes that the primary motivating force in politics and life is transactional – as his book puts it, the art of the deal. But dealing with Iran requires factoring in the power of ideology and belief. That is much harder to measure.

As this crisis has built since the turn of the year, and America assembled its armada, there have been increasing signs that the leadership in Tehran saw war as unavoidable. They engaged in talks, conscious that talks were going on last summer when Israel attacked and the US joined them.

They do not trust the US or the Israelis. In his first term Trump pulled out of the Iran nuclear deal, the JCPOA, which restricted the Iranian nuclear programme and was the marquee foreign policy achievement of the Obama administration.

There have been signs that Iran might have been prepared to accept a JCPOA mark two deal, at the very least to buy time. But the US appears to have also been demanding severe restrictions on its missile programme and its support for regional allies that oppose Israel and the US.

That was unacceptable to them, amounting to a capitulation. Giving up missiles and allies might even in the minds of the leadership make it much more vulnerable to regime change than the threat – and now reality – of attack.

Iran’s leaders will now be calculating how to ride out the war, how to survive and how to manage its consequences. Their neighbours, led by Saudi Arabia, will be dismayed by the huge uncertainty and potential consequences of today’s events.

Given the capacity of the Middle East to export trouble, the eruption of renewed and intensified war deepens the instability of region and wider world that is already turbulent, violent and dangerous.