It used to be the case that US assassination plots targeting foreign leaders were closely guarded secrets. That may be because such acts are widely regarded as illegal, both in international and domestic law. Alternatively, American reticence may have stemmed from a sense of shame. After all, the resort to extra-judicial killing – put plainly, state-sanctioned murder – does not reflect well on a country that claims to be a beacon of human liberty and democracy.

Donald Trump, different in this respect as in so many others, is evidently untroubled by such considerations. He brazenly sought credit for Friday’s assassination in Baghdad of the senior Iranian commander, Qassem Suleimani, boasting he had done the world a service. Trump claimed that an imminent attack had been foiled and that, in any case, Suleimani was responsible for the deaths of many US and allied soldiers. He said he acted to stop a war, not start one.

Trump is wrong – and dangerously so. His was undoubtedly an act of war. Iran’s response may not come immediately, but come it will and the consequences are unknowable.

Far from saving lives, Trump has recklessly imperilled them. He has increased the risk of a region-wide conflagration sucking in Iraq, Syria and Israel. And he has normalised murder of high officials as a tool of state policy, a precedent whose implications he himself might do well to ponder.

Emmanuel Macron, France’s president, rightly warns the world is now a more dangerous place. But European calls for restraint and de-escalation have gone down badly in Washington. It seems Mike Pompeo, Trump’s bully-boy secretary of state, expected the White House warrior to be showered with congratulations. That speaks volumes about Pompeo’s limited understanding of what he and his boss have just done.

The crisis now threatens to spin out of control. Casualties, human and political, could include the landmark 2015 nuclear deal with Iran. An enraged Tehran may go all out for nuclear weapons capability. Iraq has been further destabilised. The country’s humiliated government may back demands that the US withdraw its remaining troops.

That in turn could fatally compromise efforts to prevent a resurgence in Iraq and Syria of Islamic State (Isis), a joint project already undermined by Trump’s decision last year to allow Turkey to target Syrian Kurds at the forefront of the anti-Isis fighting. Attacks on Israel and Saudi Arabia by Iran’s allies and proxies appear more likely. Meanwhile, the oil price is surging, with all the global economic harm that entails.

What, at the critical juncture, is Britain’s role and interest? There can be no doubt British forces in the region have been placed at increased risk. Britain was not consulted, nor even warned, in advance of the Suleimani operation. It is obvious that Britain, closely associated in official Iranian minds with US policy, could be targeted in future revenge attacks.

So where, at this perilous moment, is Boris Johnson? Not a word has been heard from the prime minister. Apparently, he is still sunning himself in the Caribbean. Instead, the inexperienced foreign secretary, Dominic Raab, was left holding the fort – and Britain was left looking weak and irrelevant, a victim of events over which it lacks foreknowledge or influence. Is this the bold new future Johnson promised? He should remember that political honeymoons are short.

At fraught times like these, it is as well to try to keep matters in perspective. Suleimani was a fearsome, fanatical figure with much blood on his hands, a fierce opponent of the US and the west and a man who lived by violence. Unsurprisingly, he died by violence, too. It’s also a fact that the embattled Iranian regime, increasingly repressive at home, has overreached abroad, especially in Syria. Iranians are a proud and talented people, grossly misgoverned.

Yet none of this justifies Trump’s rash and cowardly act. It is wrong to murder a man, however heinous his crimes, simply because you can, in flagrant defiance of established law and the demands of justice and human decency. It is wrong to wage all-out economic warfare on any nation, inflicting needless suffering, and expect its rulers to meekly bend the knee. It would be very, very wrong of Trump to cynically use this crisis to deflect attention from his imminent impeachment trial.

Most of all, it is both wrong and stupid to pursue so aggressive and attritional a policy without the full support of key allies, without popular domestic backing or congressional approval, without due consideration for the wider international impact and, crucially, without a clear, coherent, workable long-term strategy or the will to see it through.

This, perhaps, is what is most alarming about the present situation: Trump’s lack of a plan. There was a plan, backed by Barack Obama, Europe, Russia and China, to bring Tehran in from the cold. It was working, too. It had broad international support and UN backing. It produced an unprecedented agreement five years ago. Then, in 2018, Trump casually wrecked it. This irresponsible man has not the first idea what he is doing. All that may now ensue is on him.