Why did Britain seize Grace One, an Iranian-registered tanker, the best part of three weeks ago?  The obvious answer – that it was transporting oil to Syria, and was thus in breach of EU sanctions – won’t quite wash.  After all, other Iranian vessels do the same, and we don’t usually impound them.  Furthermore, there is legal argument, since Iran is not an EU country, about the permissibilty of the move: the EU doesn’t habitually seek to impose its own sanctions on others.

America, of course, does – which takes us to the most likely cause of the seizure.  The Trump administration hopes that Iran will call off its nuclear programme if economic pressure is intense enough.  So it looks as though the Government which, on the one hand, is opposed to America’s latest tightening of its own sanctions (it reimposed these after withdrawing from the nuclear deal with Iran which Britain and other EU countries support) none the less, on the other, wants to stand with America against Iranian action in the Straits of Hormuz.  There have been two recent incidents, last month and in June, in which commercial ships have been damaged.

All this leads to another question: if Grace One was seized after pressure from America, why did Downing Street not foresee Iran’s obvious counter-ploy?  After all, kidnapping is what Julian Lewis, the Chairman of the Defence Select Committee, calls the country’s “signature move” – one that stretches all the way from the American Embassy hostage saga during the late 1970s through the continued imprisonment of Nazanin Zaghari-Ratcliffe to its latest variation on the theme: the seizure of the Stena Impero, a British-registered tanker, last week.  The Government insists that it was in Omani waters at the time.

The most likely next development is that, after the story has vanished from the headlines, Grace One will be discreetly released with its cargo, perhaps on the undertaking that it doesn’t dock in Syria, and the Stena Impero is also let go, complete with its crew.  But backbench MPs should not be deterred by the possibility of such an end from asking aggressive questions of Ministers, preferably today.  Since the navy doesn’t have enough ships near the Straits to guarantee the safety of UK-registered vessels, what is the policy – and how does the Government plan to enforce it?

Are British ships simply to be left to take their chances?  Should they be, as one senior Conservative has suggested, travelling in convoys, rather than singly?  If the Americans are willing to provide naval cover, what is the balance between accepting any such military solution and being drawn into a sanctions policy of which the Government disapproves?  Had America made such an offer already – before the Stena Impero was seized?  If we are to deploy more of our own ships (HMS Duncan will now serve alongside HMS Montrose in the Straits), where will they come from, and what are the strategic implications of any such moves?

Above all, what on earth was going on in Downing Street immediately before the seizure of Grace One?  Sources insist that it is very unlikely that any decision to impound an Iranian ship would not have been referred up from the Ministry for Defence. Theresa May has some very serious questions to answer.  Was Number Ten too preoccupied with her legacy – and the mass of policy initiatives that we have seen recently – to be across the situation in the Straits?  Mention of the Prime Minister raises another of its dimensions.  Iran would doubtless have played tit-for-tit in any event.  But this week’s changing of the guard in Downing Street will not have escaped it.

The tensions in the straits are building at a time when an untried Prime Minister is about to take office.  Boris Johnson’s history with Iran contains a blunder: he messed up over Zaghari-Ratcliffe.  He wants no distractions from Brexit.  History suggests that that he may get one: there is a long tradition of wars, or at least foreign affairs crises, flaring up in August.  With the America Government divided about how aggressively to act, and Iran’s Revolutionary Guard on the case in the Straits, the holiday season here could concide with a military escalation there.