Conflict in the Middle East has exposed the depletion of British military power, even if there was nothing that we could usefully do with it in this case.

It has been a salutary national embarrassment to discover that we barely have a functioning navy.

There would be nothing for our navy to do in the Strait of Hormuz, given that the United States, with the most powerful military in the world, cannot force the free passage of commercial ships.

In that sense, Ben Wallace, the former defence secretary, is right to describe the idea of sending British ships to police the strait as a “fantasy”.

But it took too long for HMS Dragon to reach Cyprus, and the free passage of Russian shadow tankers up the Dover-Calais strait, escorted by Vladimir Putin’s warships, mocks Sir Keir Starmer’s tough words about the Kremlin’s activities of only a few weeks ago.

The government’s missteps in handling the future of the Chagos Islands have also called into question its ability to guarantee the American base at Diego Garcia that is so important for defence and intelligence cooperation.

Meanwhile, the war in Ukraine has revealed the need for European nations collectively to shoulder more of the burden of defending their own continent – however unreasonable Donald Trump’s behaviour has been.

Sir Keir is to be commended for his leadership in rallying the coalition of the willing in support of the Ukrainian people.

He took the first step in raising British defence spending – paid for by a temporary cut in foreign aid – as part of this country’s contribution to the common effort.

The next stage, however, will be harder. Sir Keir has suggested that British defence spending should be raised from the current 2.5 per cent of national income to 3.0 per cent “in the next parliament” and to 3.5 per cent by 2035.

This would be a substantial increase but the detailed plan for how to achieve it has not yet been published, having been promised “by Christmas”.

John Healey, the defence secretary, said in the Commons last month that “we are working flat out to settle the defence investment plan, which is a plan for the 10-year transformation of Britain’s defence”, and government sources now suggest it may be published by the end of next month.

In fact, the delay in publishing the plan is understandable. These are difficult decisions, and the defence outlook is changing all the time.

The importance of the plan is that it should take us away from an obsession with spending as a proportion of GDP and focus more on the objectives of policy.

These objectives ought to include a working navy, an army of sufficient size and the capacity to innovate and to scale up production of drones, which have proved so decisive in enabling the Ukrainians to hold off the Russian onslaught.

The latest data from the International Monetary Fund shows that Britain has the smallest army relative to its workforce in Europe, except Belgium and Luxembourg.

Again, the size of the army should be determined by its capabilities rather than by arbitrary benchmarks, but the comparison does suggest that ours ought to be bigger.

That will require some difficult trade-offs. Kemi Badenoch, the leader of the opposition, said in a speech to the London Defence Conference on Saturday that the army should be increased by 6,000, and she offered her party’s support in parliament to “find the money to rearm”.

There will be no easy cuts to foreign aid to provide the next increase to defence spending – indeed, it would be desirable to restore some of the cuts that have already been made.

Public opinion will need to be prepared for sacrifices, which will be particularly difficult at a time when the closure of the Strait of Hormuz threatens a global recession. The American-Israeli war against Iran may have been unwise, but it underlined the need for stronger defences in all liberal democracies.

“This conflict is going to define us for a generation and we must respond and we will respond with strength,” Sir Keir said on Friday.

He must back up his words with action and resources. The defence investment strategy must define clear objectives and a credible means to pay for them.