What a difference a day makes. When I went to bed on 2 January what seemed to be the most important issue in the Middle East was the long-term impact of the brave – if desperate – mass protests in Lebanon, Iraq and Iran. Many were trying to extract some positive meaning: were they the precursor of a renewed popular drive for better governance in some of the key states of the region? Could they shake the stability of the Islamic Republic of Iran and its grip on the greater Levant? Or would they be suppressed and ignored, only to recur more virulently, as had happened so many times before?

When I woke up on 3 January, the world had changed. The US drone strikes that killed Qassem Soleimani and his Iraqi lieutenant Abu Mahdi al Muhandis were a salutary shock of a sort we hadn’t had for decades. Soleimani, it was thought, was off limits. Clearly Soleimani thought the same. Big mistake.

Saddam Hussein was on the lam when he was dragged out of his hole in the ground in late 2003. The murderous Osama bin Laden and Abu Bakr al Baghdadi had also been fugitives when they were killed; their followers were scattered.

In contrast, Soleimani was at the top of his game. He was the most senior external military and political representative of a major regional state, able to travel freely despite sanctions wherever he wanted. Over two decades, he was the architect of all Iran’s external strategic gains in the face of Saudi and US hostility. He was behind terror attacks in Lebanon, Kuwait, Bahrain, Saudi Arabia, Thailand, Bulgaria and the United States.

For me, as for any other British or American diplomat who had served in the Middle East during the last 40 years, this was personal. During my time as consul-general in Jerusalem, as MENA director in the FCO or as ambassador to Syria and Iraq, Soleimani was the orchestrator of Iranian subversion across the region, all aimed at Israel and the Sunni states of the Gulf but often involving us.

When British sailors were taken hostage in the Shatt al Arab by IRGC naval forces in 2007, when missiles from Sadr City targeted the British or American diplomatic missions in Basra and Baghdad, or Shia Special Groups mounted a sustained campaign of lethal roadside bombs against US and British forces, Soleimani was involved.

He was the instigator of the kidnapping of British IT expert, Peter Moore, in Baghdad in 2007 and the brutal murder of his close protection team, the aftermath of which I had to manage. He helped Bashar al-Assad defeat the Syrian uprising, the Houthis to take over the Yemeni state apparatus and then resist the Saudi military campaign in Yemen, and their various proxies, allies or subalterns throughout the region (from Lebanese Hezbollah to the various Iraqi Shia militias) extend their own domestic power.

Most recently, he had enabled or helped execute attacks on shipping in the Strait of Hormuz and the Indian Ocean, on US military drones and Saudi oil installations with very little response from other regional states or the US. He had also coordinated the brutal suppression of protestors in Iraq – building on a model he and his IRGC colleagues had developed to deal with internal dissent.

Of course, it is still possible to overstate Soleimani’s importance. Iran’s foreign adventurism is not simply driven by one man. And in the end, taking on responsibility for a hopelessly chaotic Iraq, a destroyed Syria, an impoverished Yemen and a bankrupt Lebanon – with an embattled but potentially potent Islamic State still in existence – is not exactly Metternich at the Congress of Vienna.

But Soleimani was deeply committed to the success of the Islamic revolution and unequalled in his ability to form and sustain personal relationships across the region. He had an acute grasp of regional realities and developed a highly creative approach to maximising Iran’s strengths. He wasn’t universally popular within the Iranian regime: the recent release of Iranian intelligence documents to the Intercept demonstrate that clearly. But he was very close to Ali Khamenei, the Supreme Leader. There was even talk of him being groomed for the succession. His loss is the biggest blow Iran and Khamenei personally have suffered since the late 1980s.

Khamenei will have to work out how to deal with that. He has described last night's targeting of air bases in Iraq as "a slap in the face" for the United States. He has also already moved to appoint a replacement as commander of the Quds Force, whose continued effectiveness is central to his regional ambitions. But as far as Britain is concerned, what Soleimani’s death has done is vividly and perhaps uncomfortably illuminate the real and hard choices we face as a country.

Iranian state TV has just aired video it says shows launch of missile strikes on US bases in Iraq pic.twitter.com/HhFskIkQqP

— BBC Monitoring (@BBCMonitoring) January 8, 2020

With Soleimani and Muhandis alive, you could perhaps cherish the illusion that in a reasonable world there might – with a bit of give and take – be a negotiated solution to the problem of Iran. You often see this sort of solution promoted by well-meaning, but rather unworldly commentators in Brussels and elsewhere. The catch is that under most imaginings, it is the US, Europe and the Arab Gulf who do the giving; the Iranians just take.

You could also persuade yourself – as many western diplomats in Iraq seem to have done – that people like Hadi al Ameri, the dead-eyed commander of the Badr Brigade, the most formidable of the Iraqi Shia militias and a close associate of Soleimani (with whom he served during the Iran-Iraq War) would be OK as a potential prime minister of Iraq. After all, many people had persuaded themselves that Nouri al Maliki, the machiavellian and viciously sectarian PM of Iraq from 2006 to 2014, was one of us. Except he wasn’t.

Nor is Ameri. Just watch their performances at the various mourning ceremonies for Soleimani and Muhandis and pay attention to what they say (if you understand Arabic, which too few of our diplomats do these days). They’re quite clear about where they stand. And it’s not with us.

And that’s the point. You can’t plead ignorance about what Soleimani was up to, or what he represented any more. Just read the obituaries and watch the reaction of the Iranian leadership, Hezbollah and his Iraq allies. Watch also how those who hated him in the region celebrate.

It is now being claimed in some quarters that Soleimani was only in Baghdad that day on some kind of recherché peace mission – to try and de-escalate tension between the Saudis and the Iranians. A moment’s pause to consider the lethal effectiveness of Soleimani’s region-wide network should be enough to show how preposterous this idea is. A latter-day Kofi Annan he was not.

Soleimani was a devout family man – a softly-spoken thug, for sure. But he was prepared to cajole, threaten, intimidate and commit murder if he was crossed. He had the blood of hundreds or thousands of Syrians, Iranians and Iraqis on his hands. And the Iranian leadership loved him precisely because he brought them power and influence.

All of this should simplify another choice for a post-Brexit Britain. This really isn’t about Trump: it’s about us. Are we with the EU or with the US on matters like this?

Sometimes, as when we have joined with our EU partners in trying to find ways around US financial sanctions in order to benefit Iran (so that we can “keep the JCPOA alive”), it looks as though we prefer the EU.

But there really is no EU foreign policy to speak of. There is EU development funding. There is the covert pursuit of national interest under an EU flag. And there is the usual nonsense the European left like to spout about a distinctively European contribution to peace, mutual understanding and brotherly love. In practice, all too often this means trying to do the opposite of what those “nasty” Americans do. This gives high representatives the chance to cut a dash at international conferences. Unfortunately, it doesn’t amount to much in the real world.

We shall now face a period of emotional reaction in Iran, Iraq and Lebanon. We shall also face enhanced threats to our people and our interests in the region, as last night's missile strikes demonstrates. Sure, it’s not clear that the US have a plan in response. But the people to whom this will have come as the greatest shock will be the Iranian leadership. They don’t have a plan either.

When Khamenei taunted Trump on Twitter by saying he could do nothing about the attack on the US embassy in Baghdad, he thought Trump’s America was a paper tiger. He was wrong.

Firing rockets at US military bases or at Israel doesn’t really cut it. But assassinating a senior American would be to invite retaliation on a scale Khamenei cannot imagine – or certainly countenance. And seeing US forces expelled from Iraq simply gives the Islamic State the opportunity for which they have been waiting.

It was instructive that in the heated debate on these matters over the last few days in the Council of Representatives in Baghdad, only three Sunnis and no Kurds participated. If Iraqi politics are resectarianised, it will be a disaster for Iraq but also for Iran. And if the US puts Iraq under financial sanctions as a result, the flow of dollars – on which Iran increasingly relies – will dry up again.

So the regime in Tehran, like everyone else, faces a moment of choice. Whatever else might be said about it, one decisive effect of the assassination is that it has served to clarify the options before us. And that sort of clarity is what we need in Britain.

For the last decade or more we have effectively subcontracted our foreign policy in the Middle East and North Africa to the UN and the EU, hiding behind UN special missions or empty EU policy pronouncements.

But when the rubber hits the road, as we have seen repeatedly in Libya, Iraq, Lebanon and Syria, the only things that matter are expertise, influence and power. We used to be good at all three. Now we’re not.

We need urgently to rebuild. And when we do, we need to choose sides.

If the point of foreign policy is to shape the world, then we need to be in the room when that happens. And unless we think our future is with China or Russia, that room is going to be American for the foreseeable future.

Sir John Jenkins is a Senior Fellow at Policy Exchange and has served as UK Ambassador to Syria, Iraq and Saudi Arabia, and was the Foreign Office’s Director for the Middle East and North Africa