This is the moment the government finally woke up to the enormity of the threat faced by the UK and the inability of its hollowed-out armed forces to cope.

But make no mistake, today’s decision to increase military spending is not just about increasing the number of troops, warships and fighter jets or even ensuring they can use the latest drones, satellites or artificial intelligence breakthroughs.

This is an emergency that requires the entire nation to take responsibility for – or at least an interest in – the defence of the nation and the importance of being able to deter threats.

Politics latest: PM makes defence commitment

Sir Keir Starmer signalled this fundamental shift in priorities when he told parliament: “We must change our national security posture because a generational challenge requires a generational response that will demand some extremely difficult and painful choices.”

He continued: “And through those choices, as hard as they are, we must also seek unity. A whole society effort that will reach into the lives, the industries, and the homes of the British people.”

Such a proposal is not something new.

The UK has a long history of being prepared for war.

The entirety of the Cold War era was framed around ensuring the UK had enough troops and reservists to fight a sustained conflict, supported by a vast industrial base to produce weapons and a society that was intrinsically resilient, with the ability to sustain itself with emergency food rations, power supplies and an understanding of the need to be prepared to respond in an emergency.

Back then, the threat was war – maybe even nuclear annihilation – with the Soviet Union.

Today the threat is just as stark but also far more complex.

Russia is the immediate danger. But China poses a long-term challenge, while Iran and North Korea are also menacing adversaries.

Most fundamentally though is the change in the UK’s ability to rely squarely on its strongest ally, the United States.

Donald Trump, with his resentment of shouldering the responsibility for European security, has made clear the rest of the transatlantic NATO alliance must take much more of the share of defending themselves.

He has also signalled that he may not even be willing to deploy America’s powerful military to defend every single member state – singling out those who pay far too little on their defences.

He has a point when it comes to Europe freeloading on the might of the United States for too long.

But the suggestion that European allies can no longer automatically rely on their American partner to come to their aid is enough to call into question the value of Article 5 of the NATO Alliance, which states an attack on one is an attack on all.

When it comes to deterring foes, there must be no such uncertainty between friends.

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It is why countries across Europe are being urged by the new head of NATO to rapidly ramp up defence spending and adopt what NATO Secretary General Mark Rutte has called a “war mindset”.

The UK, who along with France are the only two NATO powers in Europe to possess nuclear weapons, has a bigger responsibility than most to heed that call.

Russia’s invasion of Crimea in 2014 was not a sufficient enough alarm bell.

Eve Russian President Vladimir Putin’s full-scale war in Ukraine in 2022 failed to shake the UK and most of the rest of Europe from their slumber.

Instead, it seems the return of Donald Trump to the White House, with all the unpredictability that he brings, is the final shock that has stunned the UK into action.

Of course, defence insiders know that increasing spending to 2.5% of GDP by 2027 is not soon enough.

But this – coupled with Sir Keir’s language about the need for a “generational response” – is a landmark moment.

The beginning of the correction of a strategic mistake made by Labour and Conservative governments over years to take a “holiday from history” and fail to find credible, capable armed forces and ensure society understands the importance of defence and the ability to deter.