With Sir Keir Starmer under pressure to accelerate plans to reverse the decline, two new episodes of Sky News and Tortoise's podcast series The Wargame uncover what happened behind the scenes as Britain switched funding away from warfare and into peacetime priorities such as health and welfare after the Soviet Union collapsed.
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This decades-long saga, spanning multiple Labour, Conservative and coalition governments, includes heated rows between the Ministry of Defence (MoD) and the Treasury, threats to resign, and dire warnings of weakness.
It also exposes a failure by the military and civil service to spend Britain's still-significant defence budget effectively, further compounding the erosion of fighting power.
'Russia knew' about UK's weaknesses
Now, with the threat from Russia returning, there is a concern the UK has been left to bluff about its ability to respond, rather than pivot decisively back to a war footing.
"We've been living on a sort of mirage for so long," says Sir Ben Wallace, a Conservative defence secretary from 2019 until 2023.
"As long as Trooping the Colour was happening, and the Red Arrows flew, and prime ministers could pose at NATO, everything was fine.
"But it wasn't fine. And the people who knew it wasn't fine were actually the Americans, but also the Russians."
Not enough troops, medics, or ammo
Lord George Robertson, a Labour defence secretary from 1997 to 1999 and the lead author of a major defence review this year, says when he most recently "lifted the bonnet" to look at the state of the Army, Royal Navy and Royal Air Force, he found "we were really unprepared".
"We don't have enough ammunition, we don't have enough logistics, we don't have enough trained soldiers, the training is not right, and we don't have enough medics to take the casualties that would be involved in a full-scale war."
Asked if the situation was worse than he had imagined, Lord Robertson says: "Much worse."
'I was shocked,' says ex-defence secretary
Sir Gavin Williamson, a former Conservative defence secretary, says he too had been "quite shocked as to how thin things were" when he was in charge at the MoD between 2017 and 2019.
"There was this sort of sense of: 'Oh, the MoD is always good for a billion [pounds] from Treasury - you can always take a billion out of the MoD and nothing will really change.'
"And maybe that had been the case in the past, but the cupboards were really bare.
"You were just taking the cupboards."
But Lord Philip Hammond, a Conservative defence secretary from 2011 to 2014 and chancellor from 2016 until 2019, appears less sympathetic to the cries for increased cash.
"Gavin Williamson came in [to the Ministry of Defence], the military polished up their bleeding stumps as best they could and convinced him that the UK's defence capability was about to collapse," he says.
"He came scuttling across the road to Downing Street to say, I need billions of pounds more money… To be honest, I didn't think that he had sufficiently interrogated the military begging bowls that had been presented to him."
What to expect from The Wargame's return
Episodes one to five of The Wargame simulate a Russian attack on the UK and imagine what might happen, with former politicians and military chiefs back in the hot seat.
The drama reveals how vulnerable the country has really become to an attack on the home front.
The two new episodes seek to find out why.
The story of the UK's hollowed-out defences starts in a different era when an Iron Curtain divided Europe, Ronald Reagan was president of the US, and an Iron Lady was in power in Britain.
Sir Malcolm Rifkind, who went on to serve as defence secretary between 1992 and 1995 under John Major, recalls his time as minister for state at the Foreign Office in 1984.
In December of that year, then prime minister Margaret Thatcher agreed to host a relatively unknown member of the Soviet Communist Party Politburo called Mikhail Gorbachev, who subsequently became the last leader of the Soviet Union.
Sir Malcolm remembers how Mrs Thatcher emerged from the meeting to say: "I think Mr Gorbachev is a man with whom we can do business."
It was an opinion she shared with her close ally, the US president.
Sir Malcolm says: "Reagan would have said, 'I'm not going to speak to some unknown communist in the Politburo'. But if the Iron Lady, who Reagan thought very highly of, says he's worth talking to, he must be worth it. We'd better get in touch with this guy. Which they did.
"And I'm oversimplifying it, but that led to the Cold War ending without a shot being fired."
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In the years that followed, the UK and much of the rest of Europe reaped a so-called peace dividend, cutting defence budgets, shrinking militaries and reducing wider readiness for war.
Into this different era stepped Tony Blair as Labour's first post-Cold War prime minister, with Lord Robertson as his defence secretary.
Lord Robertson reveals the threat he and his ministerial team secretly made to protect their budget from then chancellor Gordon Brown amid a sweeping review of defence, which was meant to be shaped by foreign policy, not financial envelopes.
"I don't think I've ever said this in public before, but John Reid, who was the minister for the Armed Forces, and John Speller, who was one of the junior ministers in the department, the three of us went to see Tony Blair late at night - he was wearing a tracksuit, we always remember - and we said that if the money was taken out of our budget, the budget that was based on the foreign policy baseline, then we would have to resign," Lord Robertson says.
"We obviously didn't resign - but we kept the money."
The podcast hears from three other Labour defence secretaries: Geoff Hoon, Lord John Hutton and the current incumbent, John Healey.
For the Conservatives, as well as Rifkind, Hammond, Williamson and Wallace, there are interviews with Sir Liam Fox, Sir Michael Fallon, Dame Penny Mordaunt and Sir Grant Shapps.
In addition, military commanders have their say, with recollections from Field Marshal Lord David Richards, who was chief of the defence staff from 2010 until 2013, General Sir Nick Carter, who led the armed forces from 2018 until 2021, and Vice Admiral Sir Nick Hine, who was second in charge of the navy from 2019 until 2022.
'We cut too far'
At one point, Sir Grant, who held a variety of cabinet roles, including defence secretary, is asked whether he regrets the decisions the Conservative government took when in power.
He says: "Yes, I think it did cut defence too far. I mean, I'll just be completely black and white about it."
Lord Robertson says Labour too shares some responsibility: "Everyone took the peace dividend right through."
Building on the success of the highly acclaimed podcast The Wargame, Sky News presents The Wargame: Decoded - a one-off live event that takes you deep inside the minds of the wargame’s participants. Discover how they tackled the toughest challenges, the decisions they made under intense pressure, and even experience key moments of the game for yourself.
Click here to get tickets.
Sky News’ Deborah Haynes will guide the conversation with Sir Ben Wallace, Robert Johnson, Jack Straw, Amber Rudd, Keir Giles and General Sir Richard Barrons - real-life military chiefs, former government officials and leading experts. Together, they will unpack their experiences inside The Wargame, revealing the uncertainty, moral dilemmas and real-world pressures faced by those who must make decisions when the nation is under threat.
Join us for this unique event exploring how the UK might respond in a moment of national crisis and get a rare, unfiltered glimpse into how prepared the country truly is for war.
