In an extraordinarily blunt intervention, Air Chief Marshal Sir Richard Knighton warned that Russia's military strength is increasing and is something to fear, with Russian troops now battle-hardened after spending the past nearly four years waging a full-scale war in Ukraine.
He said the UK's armed forces would always be the first line of defence alongside the rest of the NATO alliance, but the whole of society must also play a part in rebuilding resilience.
The chief of the defence staff said it is important to be honest with "families and households" across the UK about what it means to be prepared for "an array of real, physical threats".
Serving up a sombre reality check, Air Chief Marshal Knighton said he agreed with his French counterpart, General Fabien Mandon, who said last month that France must be ready for the possibility of losing its children in a potential war with Russia.
"The situation is more dangerous than I have known during my career, and the price of peace is rising," the British defence chief said.
"Our response needs to go beyond simply strengthening our armed forces. It needs a whole of nation response that builds our defence industrial capacity, grows the skills we need, harnesses the power of the institutions we will need in wartime and ensures and increases the resilience of society and the infrastructure that supports it."
He continued: "Sons and daughters. Colleagues. Veterans. …will all have a role to play.
"To build. To serve. And if necessary, to fight. And more families will know what sacrifice for our nation means."
Most people in the UK, though, no longer have direct experience of the armed forces thanks to the peace that followed the end of the Cold War almost 35 years ago.
The military chief noted that it was 65 years ago this month since the last national service call-ups took place, and 80 years since the end of the Second World War.
But he indicated that the kind of national defence and resilience that was once second nature to UK society up until the collapse of the Soviet Union needs to return.
He said that while there was only deemed by defence analysts to be a "remote" chance of up to a 5% of Russia launching a "significant direct attack or invasion" against the UK, this "does not mean the chances are zero".
Air Chief Marshal Knighton said: "My point is that none of us can say with any certainty what the absolute risk might be. And even though simple binary statements might be easy for people to engage with, they risk instilling panic or complacency.
"What really matters is the trend. Are the chances of conflict growing?
"And here, I think the evidence is clear that the trend, from Russia in particular, is worsening, and that is the key argument for action."
Turning to the capability of the Russian armed forces, despite being locked in a major war against Ukraine, he said their "hard power is growing quickly".
"Over the past 20 years, Russia has delivered significant defence reform and investment into what were weak and hollowed-out armed forces," he said.
"The Russian armed forces are now more than 1.1 million strong, consuming more than seven per cent of GDP, and around 40 per cent of government spending, which is a sum that has more than doubled over the past decade."
By contrast, the UK army is a little over 70,000 soldiers, with the government only committed to increasing defence spending to 2.5% of GDP from 2.3% by 2027.
"We should be under no illusions that Russia has a massive, increasingly technically sophisticated, and now, highly combat-experienced, military," the military chief said.
He described how Russia is "developing new and destabilising weapons systems such as nuclear-armed torpedoes and nuclear-powered cruise missiles putting nuclear weapons in space. So it is absolutely clear that Russia's hard power is something to fear, but what about Russia's intent to use it?"
The military chief said the war in Ukraine and Russia's willingness to target its neighbouring states show it "wishes to challenge, limit, divide and ultimately destroy NATO".
But he acknowledged this threat is not yet felt in the UK as it is in countries that share physical land borders with Russia.
In a sign that he is seeking a more rapid increase in defence spending, he said Germany expects to spend 3.5% of GDP on defence by 2029, while Poland is already at 4.2%.
"And we have seen just in the past few weeks France and Germany return to a form of national service."
He continued: "In reality, other than proximity, the threat in the UK isn't really any different to the threat in Germany, for example.
"Unless we are able to raise awareness and stimulate the conversation with society about the risks, we can't expect the rest of government, society and industry to act or bear the costs."
The defence chief called out a podcast series by Sky News and Tortoise called The Wargame, which simulates a Russian attack on the UK, as an example of efforts in the media to raise public awareness.
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