A year after annexing Crimea, Vladimir Putin met various leaders involved to finalise the terms of what was, in effect, a surrender. There was dinner, with a large map of Ukraine in the middle of the table. At one point, Putin reached over and stabbed a fork on the eastern side of the map to show how much ground his troops had covered in the 16 hours they had all been talking. His gist: agree terms now, or things will get worse. No one in the West will help you. No one else cares.

I was told this story recently by a diplomat who saw it happen. Ukraine, he said, was helpless: its military was then in no state to offer serious opposition. David Cameron and Barack Obama opted out of the negotiations (saying they clashed with the D-Day commemorations in Normandy) so talks were instead mediated by France and Germany. They let Putin stitch things up in a way that loosened Kyiv’s grip on Ukraine’s eastern territories.

This 2015 deal – known as Minsk II – is now being held up in Whitehall as a reward for aggression and a case study in what cannot be allowed to happen now. But it might. Russia’s invasion is going badly: its dysfunctional army is shooting down its own planes and it has taken more casualties in the first four weeks than it did in the ten years of the Afghanistan campaign. At least a million Ukrainians are now armed and forcing Russian forces into a retreat.

The outright military defeat of Russia by Ukraine is, now, a realistic option. Putin may conclude that he can’t win, and start to negotiate while continuing the bombardment. Volodymyr Zelensky might wish to agree a deal, to end the suffering. The big fear in London is that this might mean peace terms that are dangerously generous to the Kremlin.

Over the past two decades, Putin has grown adept at persuading European leaders to give him some territory to stop him doing something even worse. It started with Georgia in 2008 where, after his invasion, Nicolas Sarkozy – then president of France – was left to broker peace. There was no fork on the table this time but instead, Putin informed Sarkozy that he intended to hang Mikheil Saakashvili, Georgia’s president, “by the balls”. He instead settled for control over South Ossetia and Abkhazia.

This is the Putin playbook. He has gained influence, even control over slices of Moldova and Azerbaijan by deploying troops. After annexing Crimea in 2014, he endured some light sanctions. But he walked away from the Minsk II talks having forced Kyiv to loosen control over the two Russian-speaking territories and making it virtually impossible for Ukraine to function as a coherent state. The Kremlin went on to arm insurgents in Eastern Ukraine who have – over the years – stoked a war that has claimed at least 13,000 lives.

Putin’s experience taught him that Europe – dependent on his gas – usually takes any excuse to look the other way. That’s why Boris Johnson is determined that, this time, it should be different. The Prime Minister is keen on “deterrence by denial” (a phrase he keeps using in meetings) by which he means building Ukraine into a military force with such top-grade equipment that Putin would not dare another invasion. Some in Whitehall are even floating the idea of the Kremlin paying damages to Ukraine (Kyiv has put costs at $565 billion so far).

But for any of this to happen, Zelensky would have to play hardball – and reject any smaller offers that Putin makes, thereby prolonging the fighting. And the killing. This is where things get tricky. So far, the democratic world has been united – strikingly so – on the need to help Ukraine defend itself. But when should the fighting stop? Here, the alliance is fracturing already, as Zelensky openly admits.

Some of his allies, he says, do want peace as soon as possible on either humanitarian or economic grounds, even if it means buying off Putin. He suspects that Germany is in this category.

Then come the liberal hawks, like Poland and the Baltic states, who want to see the Kremlin visibly punished. In the final category, Zelensky places the allies who urge him to fight on and – in his words – who “don’t mind a long war because it would exhaust Russia, even if this means the demise of Ukraine and comes at the cost of Ukrainian lives”.

This group now contains a surprising number of Brits and Americans. As one senior diplomat puts it: “If you look at all the options, our strategic interest is probably best served in a long war, a quagmire that drains him militarily and economically so he cannot do this again.” There isn’t much optimism in London about the prospects of regime change in Moscow. If Putin loses in Ukraine, he may well be deposed (even autocrats tend not to survive such humiliations) but the consensus in Whitehall is that he’d probably be replaced by someone worse.

The cost of no peace deal, too, would be substantial – in Russian lives as well as Ukrainian. Britain and Poland are both pushing for hard sanctions to stay on the whole country, not just the oligarchs. It’s not something ministers discuss openly, for understandable reasons: it would mean compounding the misery for 140 million Russians who are already suffering enough from Putin’s regime. If there is not much hope of a democratic uprising, what’s the point?

Already, Zelensky is facing criticism from Ukrainians who think he’s wrong to offer neutrality by staying away from Europe and Nato. His critics argue that neutrality means vulnerability and that the world has seen that no Russian promise can be trusted – so any deal signed by Putin would need to be guaranteed by the countries currently arming Ukraine. Their support, of course, may depend on what kind of bargain Zelensky intends to drive.

Only a few weeks ago, Zelensky looked doomed and Johnson was offering to help him set up a “government in exile” outside Kyiv. His decision to stay and fight earned him the right to agree peace on his own terms. There might be no shortage of opinions in London about what kind of bargain Ukraine should drive if he gets the chance but the decision, if it comes, will be for Zelensky alone.