Keir Starmer wants to make Britain "battle ready" with a big bang announcement on reshaping the country's defence.

The PM said the long-awaited strategic defence review will shift the UK to "war-fighting readiness" as Russia menaces Europe and Donald Trump threatens to pull the US away from defending the continent.

It's clear that the world has changed and Britain must change with it. As Mr Starmer often says, the first duty of any leader is to keep this country safe. How he plans to foot the bill is less obvious.

When the public finances are as tight as they are, every decision has a cost - and pumping billions of pounds into defence will mean other areas lose out.

The PM said in February that he would hike defence spending to 2.5% of GDP by 2027 - paid for by a raid on the foreign aid budget. He refused today to spell out how he’d pay for his aim of hitting 3% in the next Parliament.

This reticence will raise questions over whether the cash is really there. The problem is the Government has almost no room for manoeuvre.

Chancellor Rachel Reeves promised not to put up taxes on working people but also to protect the public sector from another round of austerity.

Public services have been slashed to the bone after years of Tory cuts and voters are fed up of long waits for hospital appointments, pothole-riddled roads and sending their kids to school in crumbling buildings.

Ms Reeves is also trying to keep the markets calm by sticking to strict fiscal rules banning her from borrowing cash for day-to-day spending. There are no easy choices.

Labour MPs are already on the brink of revolt over cuts to welfare and the winter fuel allowance, and how the party is going to drive down child poverty.

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Next week, Ms Reeves will set out Whitehall budgets for the next three years in the Spending Review, with fears mounting over cuts expected in non-protected departments. Paul Johnson, from the Institute for Fiscal Studies (IFS), today said that the only way to pay for increased military spending was “really quite chunky tax increases”.

But Mr Starmer and Ms Reeves believe that funding defence can deliver an economic prize - a "defence dividend" out of the extra billions being spent on Britain's security.

Firing up defence manufacturing can create economic growth along with tens of thousands of good jobs, particularly in Labour's old industrial heartlands, where Nigel Farage's Reform are trying to make inroads.

The PM chose to unveil the plans in Govan, in Glasgow, once the centre of Clydeside shipbuilding that has suffered from that industrial decline. It's not far from Hamilton, South Lanarkshire, where Reform are trying to pull off an upset in a critical Scottish Parliament by-election on Thursday.

Labour won a landslide victory promising "change" but Mr Starmer has been struggling to articulate his vision for Britain. Getting it battle ready and firing up its industrial heartlands again could be the beginning of his answer.