When the Stena Impero oil tanker was stopped by Iranian Revolutionary Guard gunboats and a helicopter in the Strait of Hormuz on July 19, HMS Montrose could not intervene. Britain’s sole frigate in the region was an hour’s steaming time away — and this confrontation has exposed how short the Royal Navy is of frigates and destroyers.

After his stunning night-time victory at the Nile in 1798, Nelson wrote, “Were I to die at this moment, ‘want of frigates’ would be stamped on my heart.” Today, however, the problem is not just a shortage of these craft but that the Navy is simply not big enough. Only about 15 frigates and destroyers are actually operational, and there are not sufficient crew for at least two of those.

Defence, security and strategy have hardly featured in the past months’ leadership and Brexit debates, except occasionally in pantomime form. Jeremy Hunt, an admiral’s son, did venture that the defence budget should double in the next decade, such were the strategic threats.

Chance would be a fine thing. Defence policy, White Papers and reviews have been regarded as toxic by parties in and out of government; defence pledges are the bolt-on nuisance item buried deep in election manifestos.

To draw some of the political sting, in 2010 the Coalition government under David Cameron instituted defence reviews every five years. Next year’s needs to be more than a quinquennial checklist. It will have to consider where Britain stands strategically post-Brexit, and what it needs to do — what is necessary and what is discretionary. It also has to take account of new technologies, information wars, new enemies and new threats.

By the end of next year the defence budget will be a shade under £40 billion annually. There is still the infamous spending gap of about £6 billion between current orders for kit and current funding. Expensive procurements, like the aircraft carriers, F-35 strike aircraft, Astute submarines, upgrades and replacements for the Army fighting vehicle fleet, are far from completed.

The services are busy — in some 80 different theatres and roles. The escorting of British-flagged and British-interest shipping in the Gulf region requires three frigates and destroyers, plus minesweepers, logistics ships, Royal Marine squads and helicopters, and a lurking submarine.

The UK has also been peacekeeping in Sudan and is to contribute 250 troops and support aircraft to French peacekeeping in Mali. Troops are on exercise supporting Nato allies in Poland and the Baltic region. Back home, an RAF Chinook has led the shoring up of the dam at Whaley Bridge — as the weather gets weirder, there is sure to be more work such as this.

Britain purports to be one of the few Nato partners spending the agreed norm of two per cent of GDP on defence (though it has to be said £1 billion a year of this is made up of service pensions and spending across the piece, including on intelligence personnel. Soon France and Germany will overtake the UK in what they spend on defence).

The headline figure seems to be beside the point. The real issue is what Britain must and can do on its own and what it must do with allies. The Gulf shipping crisis perfectly illustrates this: the UK can only offer comprehensive protection to British-flagged and British-insured shipping with the help of allies.

Other European nations say they don’t want to join so the international escort patrols, involving at least four frigates and destroyers, have to be run with the US, despite London and Washington differing widely on the Iran nuclear deal and sanctions.

We are facing new forms of conflict unimaginable a few years ago. But we can only invest in a few new techniques

Maritime security and business are vital to the UK, which gets 90 per cent of food and energy imports by sea. There are threats in a host of locations: the Strait of Hormuz, Bab el Mandeb — where the Red Sea meets the Gulf of Aden — the waters off west and east Africa, where pirates roam, and the Strait of Malacca. Nelson was right about the need for more frigates.

Put aside the 2020 defence review. The big turning point will come in 2024 — the year by which most of the current heavy equipment programmes will be complete. New threats, techniques and ways of warfare, will have arrived. “It’s the inflection point when things really do change,” says Professor Michael Clarke, doyen of academic strategists. “We will have reached ‘peak heavy metal’ for the services, and we have to work out what the UK can do in cyber, nanotechnology, and the possibilities of things such as gene manipulation and quantum technologies. We are facing new forms of conflict unimaginable only a few years ago.”

Britain, and Europe altogether, will only be capable of investing in a few of the new techniques, and for defence of their own space, rather than offence. Only the US, China and Russia seem ready and able to invest in a full spectrum of the new ways of war.

This is not to overlook the fundamental human element. In his brilliantly eccentric new book Goliath, Sean McFate, a former US paratrooper and mercenary turned academic and thriller writer, argues that whatever kinds of stateless, virtual shadow wars are to come, it is the fighter who counts.

The services still face recruitment problems — largely due to the disastrous policy of farming recruiting to commercial agencies — but things are improving.

Professor Clarke points out another weakness in the human element of UK security and strategy. “There is a huge deficit in strategic literacy — of understanding the huge reorientation of Britain in the world under way.”

The biggest worry is the deficit in ensuring a co-ordinated approach to the new reality in global affairs. The interaction of climate/environmental change with demographic change — in both population size and in movement — is set to be a game changer in security, strategy and defence.