The US will have to ignore its combative instincts if it sincerely hopes to avoid full-blown war with Iran

The diplomatic error that has led us to this unstable position is Trump’s decision to rescind the Iran nuclear deal, and the chaos that came with it. Though it’s unlikely, worse conflict may well follow

At its narrowest, the Strait of Hormuz is about the same width as the Strait of Dover. It is just as busy as the English Channel, too, and of similar strategic economic importance. Around a fifth of the world’s oil passes through this passage of water off the Iranian coast. The Strait of Hormuz, however, is also a far more hazardous environment than the waters between Dover and Calais.

The royal navy successfully managed to see off three Iranian Revolutionary Guard speed boats attacking a British oil tanker, a remarkable achievement. The guards, a sort of semi-autonomous, quasi-paramilitary arm of the Iranian armed forces, have perfected the technique of fast “swarming” much larger vessels, including American naval vessels, in order to overwhelm their defences and capture or mine them.

That HMS Montrose managed to protect the BP-chartered tanker British Heritage was not just a relief to those on board; it also avoided a further escalation in hostilities between the west and Iran in these waters, and the start in earnest of a new “tanker war”.

It was no surprise. A week ago, the UK military seized an Iranian-leased tanker off Gibraltar, supposedly breaking sanctions with a cargo of oil destined for Syria. The Revolutionary Guards pledged revenge then, and it was only a matter of time before they were going to act. This time they were unlucky, but they may not be so again.

It follows the destruction of other, third party, tankers en route to Taiwan and Japan, and the shooting down of an (unmanned) American drone. On that occasion, a retaliatory US airstrike on Iranian military installations was called off, with 10 minutes to spare, by President Trump, a rare act of self-restraint.

Tensions remain exceptionally high. The backdrop is, of course, the cold war between America and Iran that centres on Iran’s nuclear ambitions, plus its wider hostile activity, including allegedly sponsoring terrorists, against America's key regional allies such as Israel, Saudi Arabia and the United Arab Emirates. The sides are caught up, with the Russians and Turks, in proxy battles from Syria to Yemen, and the high seas are providing another theatre for hostilities to break out.

In principle, there is good reason to believe that a war between Iran and America will not break out, at least not as a positive conscious choice by either side. We have witnessed the White House’s caution over the loss of the drone. Back channels of communication between the various parties are open, notably via Oman. There is little for the US or Iran to gain from a costly conflict, one that would not only add to the death tolls in the region, but usher in a new dimension of instability and risk wrecking both countries’ economies through the effects on the oil trade. Indeed, another war in the Gulf would easily tip us into a global recession.

Yet wars sometimes start for all the wrong reasons, and through misadventure and miscalculation. The naval brinkmanship and war of words around the Strait of Hormuz and the wider Gulf of Oman region carries with them the risk of an uncontrolled escalation, and with unthinkable consequences. The original “tanker wars” in the 1980s concluded with an American cruiser, the USS Vincennes, accidentally shooting down an Iranian passenger jet, killing all 290 on board. Even that did not spark a war – but with America (and Israel) feeling threatened because of Iran’s apparent determination to acquire nuclear weapons, the context today is different.

The unforced diplomatic error that has led us to this unstable, nervous position is the decision by President Trump to rescind, unilaterally, the Iran nuclear deal, and then impose ever tougher economic sanctions on Iran and its trading partners.

As could have been predicted, this has merely convinced Tehran that its best defence against invasion and regime change is to have its own weapons of mass destruction. The ayatollahs look with some interest at the example of North Korea, where the public testing of viable rockets and nuclear warheads had brought America to the negotiating table. If America wants to contain Iran and push back its aggressive impulses, it has chosen an extremely risky way to go about it.