The change of course crystallised in a series of announcements this week.
On Wednesday, Lloyd Austin, the US Defence Secretary, convened a meeting of counterparts from more than 40 nations to cement an alliance to provide Ukraine with military support, telling reporters that "Ukraine can win".
On Thursday, the Biden administration asked Congress for approval for a US$33 billion ($51b) military aid package - a sum equivalent to half Russia's entire annual defence budget.
On Friday, Congress approved a Lend-Lease act for Ukraine modelled on the plan that helped Britain and the Soviet Union defeat the Nazis.
The American moves came alongside pledges of heavy weapons and assistance from other allies. Western governments, argues Keir Giles, an author specialising in the Russian military, are realising: "Russia is not going to stop until it is stopped."
In other words, Ukraine's allies not only believe it could win, but that it has to. But how?
The shortest path to victory lies across the battlefield. In the imagination, it goes something like this: While resisting the Kremlin's offensive in Donbas, Ukraine re-equips and retrains on the armour, artillery and air-defence systems pouring in from Western allies.
As the Russian offensive peters out, Kyiv launches its counter-offensive. Ukrainian tanks rout Vladimir Putin's over-extended troops and chase them back to the border. The Kremlin, its army shattered and arms industry throttled by sanctions, has no choice but to come to terms. But that, says Mark Galeotti, a veteran Russia watcher, is unlikely.
The Russian army may have botched its blitzkrieg. It may have problems with morale, logistics and leadership. But Moscow still has more men and guns, and has historically been very tenacious on the defensive. If it digs in on captured territory, the onus would be on the Ukrainians to find the 3:1 numerical advantage required for offensive operations.
If Vladimir Putin formally declares war - a step many expect on May 9 - he could raise conscripts from a population three times the size of Ukraine's. Even with its new kit, a Ukrainian counter offensive to retake the swathes of Kherson, Zaporizhya, Kharkiv, Donetsk and Luhansk Oblast would be a tall order.
"The odds are we are not heading for a situation in which either Russia or Ukraine can deliver a knockout blow. So the question is where the lines are: will it freeze or not?" Galeotti said.
The second route to the Truss version of victory is a combination of military and economic attrition that becomes unbearable for Russia.
"That is about Russia's capability to re-arm under sanctions. But also it is to lead to this revolution in the Russian elite's mindset - that they cannot wage a war against the united West. They have never waged a war against a united West that they have won," said Orysia Lutsevych, director of the Ukraine Forum at Chatham House.
How long it will take for that realisation to dawn in the Kremlin, no one knows. Truss spoke about a "long haul". Some Western officials speak in terms of years not months. In the meantime, the prospect is of a repeat of the stalemate that took hold in Donbas after 2015: a fortified front line snaking hundreds of miles across the steppe, slowly calcifying into a de facto border.
And time can work for both sides. While the West hopes to grind down Russia economically, the Kremlin will be hoping to hold out until cracks appear in the Western alliance.
"We see some alignment on those objectives between Eastern Europe, the UK, US and Canada, but there is still a gap with what Germany and France think," Lutsevych said.
"They have another view that we should not corner him or threaten Putin too much militarily with defeat. The big question is, how do you defeat a nuclear power?"
It is a reasonable question. How would Putin react to the prospect of defeat remains a central concern. He has made reference to using nuclear weapons, but exactly when he might resort to them remains ambiguous
Margarita Simonyan, a loyal propagandist, suggested the threat of defeat would do it.
"Either we lose in Ukraine or the Third World War starts," she told Russian Television.
Many Ukrainian officials dismiss such threats.
"People are saying we should be careful not to win too much," said Vadym Prystaiko, the Ukrainian ambassador to London.
"But we have been fighting a nuclear power for eight years. What do you want us to do?"
And there remains some obfuscation about where Western war aims end. President Volodymyr Zelensky has said there could be no ceasefire until Russian forces had retreated to their positions before the February 24 invasion.
Last week unnamed Western officials described a similar goal as the "minimum"
objective.
Truss has gone further. She seems to imply Ukraine must also take back both the parts of Donbas occupied by Russia and its proxy separatist republics in 2014, and Crimea, which Russian annexed the same year. That, many believe, may not be realistic.
