While holidaying in Norway recently, I spent some time reading about the country's experience during the Second World War.

What struck me most acutely was how quickly events unfolded in the spring of 1940. The Norwegians were brave – but courage alone could not compensate for a lack of military preparedness.

When the Nazi troops arrived on their shores, Norway simply did not possess the means to repulse them.

Its experience resonates with me today because we, too, must face an uncomfortable truth: Russia is at war with Britain – and it has been ever since we became the first Western country to supply lethal anti-tank weapons to Ukraine in January 2022, a month ahead of Moscow's full-scale invasion.

And yet too many British politicians refuse to acknowledge this inconvenient truth.

If proof were needed however, it arrived on Monday in a British courtroom, when it emerged that two men convicted of arson attacks on two homes belonging to Sir Keir Starmer, together with a car he had once owned, had been hired by Russian operatives.

In other words, a hostile foreign state sought to burn to a cinder three assets linked to the British Prime Minister.

Had China attempted such an act, it would have dominated the headlines for months.

Had Iran done it, ministers would rightly have described it as an outrageous attack on British sovereignty.

But because it was Russia, the incident passed largely unremarked on by politicians of all hues. Worse was to come.

The very next day, we learned that a Russian warship had fired warning shots at a British sailing boat, crewed by two retirees, in the English Channel.

Do not believe for a moment the claim by the Russian Defence Ministry on Tuesday that this was simply a routine maritime warning. Less than 48 hours earlier, Royal Marines had boarded an oil tanker called Smyrtos that was suspected of being part of Russia's shadow fleet of vessels helping it to circumvent sanctions on its oil exports.

So this was a clear message from Moscow: interfere with our economic lifelines and there will be consequences.

And this was only the latest episode in the Kremlin's campaign against our island.

In recent years, Russian state actors have engaged in a systematic effort to map our network of critical undersea infrastructure, including telecommunication cables and energy grids, a move clearly designed to give them the intelligence they will require to sabotage it at some future date.

Meanwhile, the devastating and costly cyber-attack on retail giant Marks & Spencer last year almost certainly had emanated from Moscow.

All the while, our politicians have continued to keep their fingers in their ears and blindfolds over their eyes.

Only last week, the Secretary of Defence John Healey resigned his post in protest at the Treasury's unwillingness to commit more resources to expanding our military at a time of growing danger.

As he emphasised in his resignation speech to the Commons this week, defence spending falls 'well short of what is required'.

How right he is. Russia's vast military-industrial complex produces more than seven million drones every year while in Britain, even after a decision to increase their manufacture, our production rate of 120,000 per annum remains a tiny fraction of that figure. It's the same story when it comes to manpower. Russia currently fields around 700,000 troops on the Ukrainian frontline alone. Britain's entire full-time Armed Forces number roughly 130,000 personnel.

These figures should concern every citizen because they tell the story of a nation that has spent decades assuming that major wars belonged to history.

That assumption is no longer sustainable – and if we do not act now, and act decisively, we face ever greater peril.

The first step is to engage in an honest discussion with the electorate about the reality of the threat we face, and what it means for us.

The Government must make it clear that we need to restructure our public expenditure, pivoting away from welfare to defence.

At the same time, we must take decisive action against our enemy by hitting them where it hurts. Russia's war machine runs on energy exports and much of its crude oil is sold to India, China and Turkey, who refine it and sell it on.

Together with our allies, we should make it clear that continued participation in this trade will carry severe consequences in the form of secondary sanctions.

Simultaneously, we should seize the $300billion in Russian state assets currently frozen in Western banks and spend them on Ukraine's defence.

I have spent years studying Vladimir Putin and I know that he will stop at nothing.

If we fail to act, there will be more sabotage, more intimidation, more attacks on infrastructure and more attempts to undermine democratic societies from within.

Sir William Browder is head of the Global Magnitsky Justice Campaign and author of Red Notice, A True Story Of Corruption, Murder And How I Became Putin's No 1 Enemy.