Forty years ago today, Margaret Thatcher learnt what happens when you fail to deter potential aggressors. Argentina invaded the Falkland Islands. Our tiny garrison of Royal Marines surrendered and was made to lie on the ground. Britain was humiliated. The prime minister’s fall was widely expected.

Eleven weeks later, it was Argentina’s turn to surrender. The skill and courage of the Armed Forces and the leadership of Mrs Thatcher had turned defeat into victory.

It is not only this anniversary that prompts what follows, but also the situation in Ukraine today.

Last year, the BBC commissioned me to present a two-part documentary about the relationship between Mrs Thatcher and Ronald Reagan. The film was scheduled for about a month’s time, but the BBC has decided, in view of current events, to bring it forward. The first part appears on BBC2 this Sunday at 9pm.

Although the word “Ukraine” does not feature in the programme, the corporation’s change is justified. The story of the Reagan/Thatcher relationship has a highly topical application. It is about how much allies can do if they know what they believe and have established the trust to act together against enemies.

Usually, world leaders become acquainted only in office. Mrs Thatcher and Reagan were different. They first met four years before she reached Downing Street, six before he made it to the White House. Their friendship was the stronger for having been forged in adversity.

Theirs was a lonely revolt against the weakness of the 1970s Western establishment in the face of the imperial assertiveness of Soviet Russia. When Mrs Thatcher was still in opposition, the Russians’ Red Army newspaper put her on the map. She’s the Iron Lady, they said in sexist mockery. She grabbed the insult and made it an accolade.

Once in power, she and Reagan succeeded in installing cruise and Pershing missiles in Western Europe – overcoming political agony for many Nato members. With nuclear resilience achieved, they could bargain with the Soviets from strength.

It was Mrs Thatcher who made the first important opening to Mikhail Gorbachev, even before he had become the Soviet leader. Reagan trusted her assessment of him and took matters further. By the time Reagan left office in January 1989 (she stayed on until November 1990), the end of the Cold War was nigh, though few expected it so quickly.

The Falklands experience affected Mrs Thatcher deeply. First, it taught her no end of a lesson: Argentina would never have invaded if her Government had made clear it would defend the islands.

Second, it taught her to work with her key ally under extreme stress. She was shocked, early on, that their interests diverged. As its name suggests, the United States of America cares what happens throughout its continent, whereas Mrs Thatcher focused only on what Reagan called “that little ice-cold bunch of land down there”.

In the end, through friendship, and by navigating a diplomatic process which could have handed the Falklands over to some internationally policed “interim administration” against the wishes of the islanders, Mrs Thatcher convinced Reagan that the Anglo-American alliance was paramount: a British victory would greatly benefit the much wider struggle.

It moved so fast. As late as May 31 1982, Reagan rang Mrs Thatcher begging her to cease fire rather than defeat Argentina outright. She dismissed this out of hand. (She: “Listen, Ron. They were the aggressors. They asked for this.” He: “Yes, Margaret. Ah … er, er, er … yes, yes, yeah.”) A week later, he was riding with the Queen at Windsor and addressing both Houses of Parliament in the Royal Gallery. It was Ron, not Margaret, who made the key point. Referring explicitly to British troops in the Falklands, he said: “Those young men aren’t fighting for mere real estate. They fight for a cause – for the belief that armed aggression must not be allowed to succeed.”

Among those who took careful note of her victory, and the American alliance that helped deliver it, was Soviet Russia. Its judgment of the seriousness of Western intent changed. That intent, manifested militarily, economically, politically and diplomatically, won the Cold War. And, as Reagan and Thatcher had always publicly, vividly advocated, the victory was intended not only for the West which they led, but for the peoples of the East whom they sought to liberate.

We all now know that victory was partially squandered. We began to live lives luxurious enough to afford a culture war, thus emboldening Vladimir Putin to start a real one. Oblivious of threat, Western leaders this century have lacked the wisdom to forge a bond as strong as that of Thatcher and Reagan. Alas, Barack Obama, the only 21st-century Western leader of global vision, had the wrong one.

However, this column should not be one of the many moans you will have read about Western decadence in recent weeks. I should like to consider the idea that we – particularly Britain – may be showing some signs of getting these things right.

I talked this week to Air Marshal Edward Stringer, recently retired and one of the most clear-minded military thinkers. He made the following arresting statement. Operation Orbital, he said, “is the most cost-effective British military operation since the Falklands”.

Orbital began in 2015 after Russian occupation of the Crimea the previous year. Since then, it has trained 22,000 Ukrainian armed forces in their country, never with more than 100 British service personnel present at any one time. The underlying idea has been to help the Ukrainians out of the Soviet military mindset they inherited and replace it with a Nato one. Instead of orders coming down rigidly, untrustingly, from far away, emphasis is placed on “mission command” – initiative in the field, agility, and an overall intent which commanders imbue in subordinates (a skill in which Nelson was a master).

The latest technology, of course, is also a vital part – much nimbler, for example, to have drones that can destroy tanks than to have lots more tanks. Cyber, too, is central, as is the US/UK innovation of speedy release of intelligence about Russian activities, as is sophisticated multi-platform, multi-targeted propaganda. Given that the total Nato defence budget of all members is about $1.2 trillion, the cost of Operation Orbital is proportionally minute. The Ukrainian defence budget is a tenth of the British one. Yet the might of Russia really does seem to have stalled. The word “cost-effective” seems apt. That is why President Zelensky has been warmer towards Britain and Boris Johnson than to any other major ally.

Our Nato co-workers in such efforts have been the United States and Canada, not France or Germany.

Putin’s army, by contrast, finds its conceptual, moral and physical aspects all at odds – how can soldiers charged with “liberating” a people spend most of their time shooting, starving and bombing them? The Ukrainians have the classic coherence that comes when almost the entire working male population wants to defend the nation and the armed services have been preparing for several years.

Last year’s Integrated Defence Review described Russia as “an acute military threat”, which some people said was silly. It also saw China as ultimately by far the more serious challenger. Both judgments are surely right, and the one is now more clearly linked with the other. Belatedly, cautiously, Nato is developing a much clearer intent. How admirable of the Ukrainians to inspire us by example. How much more we need to do for them.