Few people in the west doubt that Ukraine is fighting a just war. Russia’s invasion was entirely unprovoked. Whatever complaints it may have had about Nato expansion or Ukraine’s mistreatment of Russians in Donbas, nobody had attacked Russia, and nobody was planning to. Vladimir Putin launched a straightforward war of aggression and territorial conquest.

It follows that supporting Ukraine is the right thing to do. But it is not at all clear that the kind of support we are giving (and not giving) is the right way to go about preserving the Ukrainian nation.

The longer this war rages on, the more Ukrainians will flee their homeland, and the more devastation will be wrought upon their homes, cities, industry and economy. Yet the west’s current approach of supporting Ukraine’s war aim of defeating the aggressor, and providing arms for that purpose while pointedly avoiding direct military intervention, is guaranteed to prolong the war. Russia’s progress may be slowed, but it’s highly unlikely to be stopped, far less pushed out of Ukraine, and in the meantime the grinding destruction and hideous war crimes will continue.

No day goes past without some senior western politician proclaiming that Ukraine will be “successful” and that Russia is “failing”. This is certainly morale-boosting. But it is clearly nonsense.

The fact is, as time goes on, more towns and cities are destroyed and then fall to the Russians. In two months, the area under Russian control – originally just the breakaway parts of Donbas – has grown to perhaps five times the size. If Russia continues to suffer “defeats” at this pace, then in another two months the entire south of Ukraine will be in ruins, cities such as Odesa will resemble Mariupol, and thousands upon thousands more Ukrainians will have died.

Worse, as the war goes on, and more towns are destroyed, it becomes less likely that Ukrainians who have fled to other countries will ever return, because they will have no homes or workplaces to come back to. How many citizens of Mariupol will ever return? If Russia’s aim was to exterminate the Ukrainian nation, then the west’s approach is helping to do just that.

Surely, if the lives of Ukrainian people are our concern then the west has to do something to stop the war – now. Encouraging the Ukrainians to continue, however just their cause, is merely making their country uninhabitable.

The trouble is, there are only two ways to stop the war quickly, and neither is palatable to most western leaders.

One would be for Nato to enter the war and make a quick, massive and decisive strike to cripple Russia’s invasion forces. Unlike with Russia’s actions, it would have every right under international law to do so. When Putin intervened in Syria, he very carefully framed this as a response to a request from Syria’s legitimate and internationally recognised government. The west could do the same in Ukraine. Putin himself has no such justification for his invasion.

The risk involved in this – of a third world war – is obvious, and it’s why the west refuses to intervene directly.

The other option is to persuade Putin to implement an immediate ceasefire, by inviting Russia to comprehensive peace talks. Western leaders are disinclined to parley with a butcher such as Putin. But they did it with Serbia’s Slobodan Milošević, only months after the massacre at Srebrenica, and the result was the Dayton agreement that put an end to the war in Bosnia in 1995.

To get Putin to the negotiating table at all, everything would have to be up for discussion – including Ukraine’s borders, Russia’s age-old security concerns, perhaps even the very logic of basing today’s international frontiers in that part of Europe on what were internal borders in the USSR, drawn up by communist leaders precisely to prevent Soviet republics and regions from being viable independent states. The outcome of the talks does not need to be predetermined. The important thing is to talk rather than fight.

Western leaders cannot bring themselves to broach these matters, which would seem to reward Putin for attempting to redraw the map by force. They would rather fight – or more accurately, let Ukraine fight, in the hope of defeating Russia. But if one thing is certain it is that Putin will never accept defeat. He is already too deeply invested in this war to back off with nothing to show for it. If western leaders think that their arms-length encouragement of Ukraine will bring about a Ukrainian military victory, then they are fatally misreading Putin’s intentions and resolve. For Ukraine’s sake, we need to stop him now, one way or the other, before nothing is left of the country we want to protect.

• Angus Roxburgh is a former BBC Moscow correspondent and former consultant to the Kremlin. He is the author of The Strongman: Vladimir Putin and the Struggle for Russia and Moscow Calling: Memoirs of a Foreign Correspondent

The Guardian, UK