In the months leading up to Russia's full-scale invasion of Ukraine, I was working at the Ministry of Defence as a senior staff officer. As we saw the Russian build-up, we war-gamed Putin's possible strategies in order to advise our politicians on their options. It was very hard to see how we could stop him, if at all.
In hindsight, the Russian dictator had set his mind on a quick war and a marvellous victory parade in Kyiv, and the only variable was when it would all start.
As the inevitability of military action became clear to us, the options for the West narrowed. But we needed to do whatever we could, short of joining in the war, to help the Ukrainians resist occupation. That way, we would help them inflict a deep cost on Russia for invading a neighbouring country.
After all, if it was that easy to swallow up Ukraine, might the Russians get greedy and roll on through Moldova, to bits of the Balkans and the Baltic states? I certainly thought so.
As it turned out, when the shooting started, Ukraine mattered more to Ukrainians and to Europe than the Russians had predicted. Their spirit of uncompromising defiance spurred something deep in the British people.
I remember the large crowds of quiet but resolute citizens demonstrating solidarity for Kyiv. They demanded action and leadership from their Government, and across the country people hoisted up the blue and yellow flag of Ukraine, opened their homes to refugees and raised money and supplies to send to the new European eastern front. We cared – and were right to.
The trajectory of Britain's response was thus set. From the political to the tactical, we were ordered to do what we could to help.
On my first visit to Kyiv shortly after the full-scale invasion, the Ukrainian generals were clear that this was their war to fight. Our role was to gather as much international diplomatic, military and economic support for them so they could defend against the invasion, get as much of their country back as possible and make the Russians pay a high price for what they had started.
British soldiers from the Duke of Lancaster's Regiment clear buildings on exercise in Estonia
British special forces practice their rapid deployment techniques with a Chinook helicopter
They wanted to emerge from the war stronger, closer to Europe and more aligned with Nato, so that this would never happen again.
Simply put, they wanted at last to break away from the centuries-old grip of Russian imperial ambition, as so much of central and eastern Europe had done at the end of the Cold War.
The reason British support mattered wasn't just out of principle. It was the realisation that this war was about more than Ukraine.
Putin's strategic design was much greater. It was, and remains today, the collapse of Nato, division within Europe, and ultimately restoring dominion over those states that were once part of the Soviet Union.
We, and the West generally, are absolutely in the crosshairs of this. It's a deal on their terms, or no deal at all.
This is not going away however the war in Ukraine comes to an end. Unless something changes, I believe we are on a collision course with a Russia that is on a war footing, that is replenishing its lost equipment and that is re-arming fast. Neither the terrible price his troops have paid with their own blood on the battlefields nor the extraordinary resolve and fighting power of the Ukrainians to keep going has made Putin think twice about the cost versus the gain. This war still really matters to him, and it's in the balance. Wars always are, until they end.
In response, with all that it means for our collective future, the British Army – alongside the Royal Navy, Royal Air Force and the Cyber & Specialist Operations Command – has been ordered to get ready to 'war fight' alongside the rest of Nato.
As the military professional with responsibility for the British Army, I must think about the worst-case scenario.
Captain Hamish Mackellar, of 2nd Battalion The Royal Anglian Regiment, looks for enemy troops on exercise in Estonia
British troops from Duke of Lancaster's Regiment patrol through snow in Estonia
My almost singular focus for the Army has been on modernising and boosting the fighting power of our land forces as quickly as possible, so our soldiers can fight and win their battles if called upon. That's what the Strategic Defence Review recommended and we're getting on with it, informed by almost daily insights drawn from the war in Ukraine.
But as my Ukrainian opposite number warned me when I saw him some months ago, it's not just the front lines that matter when you are dealing with this Russia.
Putin will only take us seriously when he sees our factories producing at wartime rates. And that's why I believe so strongly in the need to rebuild our own national arsenal and reconnect society with its Armed Forces.
It's time for the economic regeneration of traditional defence industries, and also a good moment to create new ones for robotic and autonomous systems, artificial intelligence and cutting-edge software. These are the new tools of war.
The urgency could not be clearer. Just ask yourself: If you knew now that our soldiers would be involved in large-scale combat operations in 2027, what would you be doing differently – and why are you not doing it?
I have been amazed at how many British businesses, service providers and manufacturers have come to us to ask how they can help. That spirit of generosity and an inclination to serve Britain in whatever way they can, by bringing their skills and expertise to bear, or taking a risk with their cash flow, is inspirational.
I can assure you that it has been matched by bottom-up innovation and creativity from our soldiers, too. This is all giving them the confidence to know their country has their backs if it comes to combat. They will be ready in mind and body, no question, but it needs to be more than that.
Russia started this war by invading Ukraine, and it seems to me only they can decide to stop it.
Ukraine is showing extraordinary resolve and ingenuity, and we need to keep helping them do that.
We can also signal to Putin that, having failed to achieve his mission after four years of what was meant to be a short excursion to steal a country, if he thinks it will be any easier to steal a bit of Nato territory, he is even more stupid than we thought. So don't start a war with us, or any of our partners or allies.
We will never give up what matters to us. Our future will be on our terms, and ours alone.
