In both cases, the administration has demonstrated a willingness to surprise by unilaterally leveraging US strength in the pursuit of long-term outcomes, despite considerable short-term risks and without wide consultations. As Ronald Reagan showed in the 1980s with his strategy vis-a-vis the Soviet Union, such aggressive unilateralism can work. But it is best used selectively and sparingly.

In seeking to address long-term US (and European) grievances against certain Chinese trade practices, the Trump administration decided to abandon the traditional approach of seeking redress through existing multilateral institutions such as the World Trade Organization. Instead, it opted for what game theorists call a non-cooperative approach, imposing harsh tariffs on Chinese imports and then threatening even more should China retaliate. By weaponising what is traditionally an economic-policy tool, the US has been able to pursue national-security objectives alongside economic and financial goals.

So far, at least, the underlying calculation has worked for Trump. Just like his unilateral push to overhaul the North American Free Trade Agreement, he has shown a willingness to tolerate some damage at home in the hope that the damage inflicted on the other parties would be far greater and force them to make concessions.

Pressing an adversary into a corner in the hope that it will make a mistake is an old strategy

The Reagan administration perfected this approach when it embarked on an accelerated arms race with the Soviet Union (or what Reagan labelled the “evil empire”). In ratcheting up defence spending, Reagan leveraged the US’s economic and financial strength in the knowledge that the Soviets could not possibly keep up. In the end, he secured not only a narrow tactical victory but a wider geopolitical triumph, eventually culminating in the Soviet Union’s collapse.

Pressing an adversary into a corner in the hope that it will make a mistake is an old strategy. And as with any strategy, it comes with potential costs and risks. In Reagan’s case, his administration had to bear the economic and political costs of creating large budget deficits, as well as the risk of a serious military confrontation with the Soviets.

In the case of Nafta, the Trump administration ran the risk that retaliation by Canada and Mexico would result in beggar-thy-neighbour outcomes for all. After an initial attempt to do so, however, they made many of the concessions the US wanted and that are now included in the US-Mexico-Canada Agreement (USMCA). China, however, initially reacted differently, and may have overplayed its hand. By prolonging trade tensions with the US through an extended tit-for-tat escalation of tariffs, the authorities allowed the economy to slow and prompted many companies to start diversifying their supply chains in an effort to reduce their reliance on China. The new mantra among a growing number of those operating in China reflects this: “In China, for China”.

The immediate effect of these developments has been to complicate China’s effort to implement the structural reforms needed to maintain its historic (if not unprecedented) rise. But over the longer term, the further-reaching effects of the trade war could increase the possibility that China will become ensnared in the middle-income trap, like so many developing countries before it. If that happens, the US will have managed to avert the global economy’s full evolution from an American-led order to a bipolar one.

All of this takes us to Iran, where the Trump administration’s recent confrontation seems to be following the same playbook. The pattern is clear: the administration takes a surprise step that its predecessors might have considered but never pursued; it does so without wide internal and external consultations. The immediate result is a significant spike in tensions, with third countries (including allies) raising concerns about America’s unilateralist turn. The adversary (in this case Iran) issues a response that, while less severe, leads it also to make a tragic mistake (the accidental downing of a civilian passenger flight). Now, even the European countries that had long sought to salvage the 2015 Iran nuclear deal are accusing Iran of violating that agreement.

How this conflict will play out remains to be seen. But it is already clear that the US has made some gains, and that the greatest immediate risk – an outright war or a destabilising asymmetrical conflict – has been avoided, at least for now.

Again, however, this is not to suggest that the strategy of pressing one’s relative strengths is always advisable. Excessive reliance on aggressive unilateralism risks dismantling an international architecture that has served US interests well. Moreover, the Trump administration’s actions, if pressed too hard, could force third countries to make choices that run counter to US interests. Witness, for example, some countries’ continued willingness to deepen their economic and financial relationships with China through its “Belt and Road Initiative,” despite US objections.

In the end, aggressive unilateralism is not an approach that can be applied as a general rule. It should be used in a highly selective and infrequent manner, and only after a careful assessment of the costs and benefits. Done right, it can help achieve targeted gains while containing collateral damage. But if it is abused, far-reaching unintended consequences could follow, implying ever-higher costs over time.

Mr Johnson said he recognised that Mr Trump saw the accord as “flawed” and suggested he could renegotiate it.

Mr Rouhani warned that “all Trump has done is break promises”.

He also criticised the UK, France and Germany for triggering the nuclear deal’s dispute mechanism after Iran breached key commitments.

President Trump abandoned the nuclear deal in 2018 and reinstated US sanctions on Iran to try to force it to negotiate a new agreement that would place indefinite curbs on its nuclear programme and also halt its development of ballistic missiles.

The five remaining parties to the deal – the European powers plus China and Russia – want to keep it alive. But the sanctions have caused Iran’s oil exports to collapse, the value of its currency to plummet, and sent its inflation rate soaring.

After the Trump administration increased the pressure on Iran in May 2019, the country responded by gradually lifting all limits on its production of enriched uranium, which can be used to make reactor fuel but also nuclear weapons.

Iran announced that the final limit had been lifted earlier this month, days after top Iranian general Qasem Soleimani was killed in a US drone strike in Iraq. Iran’s armed forces also fired ballistic missiles at two Iraqi bases housing US forces in retaliation.

When US President Donald Trump called on the UK, Germany, France, Russia and China — the other nations signatory to the Joint Comprehensive Plan of Action, otherwise known as the Iran deal — to join him in walking away from the JCPOA, he was asking his European allies to do far more than isolate Iran.

The Iran deal, which was signed under the auspices of the European Union, is the single biggest foreign policy achievement in the EU’s history.

It was the EU that drove efforts to get all of the significant parties to the table. In doing so, it not only encouraged Iran to engage with the West, but it crucially created a forum in which the EU could start to navigate what is now its chief aim in foreign policy. “The EU’s top priority is balanced relations between the big two: China and America,” said Steven Blockmans, head of foreign affairs at the European Center for Policy Studies.

Europe’s China problem is acute. The continent’s stagnating economies benefit from Chinese investment, but that often comes with the potential security risk of allowing state-owned Chinese companies like telecoms giant Huawei to operate in Europe.

For its part, China is only too happy to cement its position as a major influence in Europe, home to some of the largest economies on the planet.

Historically, China and Iran have good diplomatic relations. These improved after the signing of the JCPOA, as Chinese investment in Iran increased and continued even after Trump withdrew from the nuclear deal.

Something that China and Iran also have in common is poor relations with Trump. He has tried to marginalize both countries by entering into a trade war with one and imposing sanctions on the other.

Meanwhile, Iran has friendly relations — diplomatically and militarily — with Russia, another state that the EU must maintain a complicated balancing act with. Many EU nations rely on Russian investment and natural resources, while major European financial centers, including London, have seen huge investments from wealthy Russians looking to get their money out of Russia.

However, in recent years, Europe has imposed financial sanctions on Russia, following Russian interference in European nations, ranging from waging misinformation campaigns to the state-backed assassination of Russian dissidents. And Western European nations have been among the most vocal in standing against Russia’s illegal annexation of Crimea in the east of Ukraine.

In asking his European and NATO allies to have a bigger involvement in his standoff with Iran, Trump is essentially asking them to make a choice: stay on good terms with new friends in Beijing, Moscow and Tehran; or fall in line behind its old ally, despite the fact it’s currently led by a man who is considered worryingly erratic by most European diplomats and might not even be in office after this year.

Then again, he might be. Europe has to have a long, hard think about what the implications of a second Trump term could be. Four more years, sure. But what would it mean for the long-term politics of the most powerful nation on earth, and can America ever go back to how it was before?

Complicating the decision for the EU, at least, is the fact that one of its most powerful members is leaving the bloc in less than a month. “The UK is leaving the EU at a time when Trump is trying to renegotiate the transatlantic relationship,” said Mark Leonard, director of the European Council on Foreign Relations. “There’s a danger that as Britain leaves the EU, it puts getting trade deals above all else.”

Sitting at the top of the UK’s hit list in terms of trade are the EU and the US — and the latter is a political priority for London. But most trade experts believe that a comprehensive deal with one sinks the chances of doing so with the other.

A good example of how difficult a balance this is for UK Prime Minister Boris Johnson can be seen in how he has both supported the US in its strike that killed Iranian military chief Qasem Soleimani, while also telling Tehran that he has every intention of continuing to support the JCPOA. It’s unclear how long maintaining both positions remains viable — especially in the wake of the deaths of four Britons among the 176 who lost their lives in the Tehran plane crash that Iran has admitted responsibility for.

Mr Trump, in a televised address from the White House, said no US or Iraqi lives were lost in the attacks and the bases suffered only minimal damage.

The Irbil and Al Asad bases were attacked early on Wednesday local time.

Iran said it acted in retaliation after the assassination of top Iranian General Qasem Soleimani last week.

The US drone strike targeting him and Iran-backed militia figures in the Iraqi capital Baghdad was a major escalation amid already deteriorating relations between Iran and the US.

Iran’s Supreme Leader Ali Khamenei described the missile attacks as a “slap in the face” for the US and called for an end to the American presence in the Middle East.

On Wednesday evening at least two rockets fell in Baghdad’s heavily fortified Green Zone, where the US embassy is located. There were no reports of any damage or casualties.

The US attack on Soleimani killed members of pro-Iranian Iraqi militias, who have also said they would seek revenge.

But Mr Trump appeared to draw back from a threat to target Iran’s cultural sites should Tehran respond militarily.

Iran was burying Soleimani on Tuesday, but a stampede at the funeral killed 50 people and injured 200 more.

In remarks at the Oval Office, President Trump also addressed the issue of Iraq, saying a US withdrawal of troops would be the worst thing for the country.

His comments came in the wake of a letter, which the US military said had been sent in error, to Iraq’s prime minister, apparently agreeing to a request by Iraqi MPs to pull troops out.

Soleimani’s killing has raised fears of a conflict between the US and Iran.