Ukraine’s foreign minister, Dmytro Kuleba, was flying home on a Turkish Airlines flight from New York when Russia invaded. He had just been welcomed into the White House by President Joe Biden. It felt like he had been given a diagnosis of Stage 4 cancer, he thought. The warm handshakes, the empathetic smiles were meant to be final farewells — for him, and for his country.

In Washington, and in most European capitals, no one expected Ukraine to survive in February 2022. The CIA director, William J. Burns, had secretly flown to Kyiv at Biden’s request, warning Ukrainian president Volodymyr Zelensky that Russia was planning to assassinate him.

“They spoke about the physical liquidation of our leadership, about the creation of filtration and concentration camps,” said Zelensky’s national security adviser, Oleksiy Danilov. “But what could we do? We kept asking: give us weapons. But they didn’t really give weapons to us.”

As the United States shut down its embassy in Kyiv ahead of the invasion, it did ship some weapons to great fanfare, such as Javelin antitank missiles. But the quantity was puny: only about 90 Javelins, according to Danilov. Boris Johnson, then the British prime minister, while also briefed about the hopelessness of the Ukrainian cause, had overruled internal objections and authorised a heftier load of about 2,000 NLAW missiles. Still, those were weapons best suited for a guerrilla campaign, not a conventional war.

At the time, Ukraine’s military leadership, under General Valery Zaluzhny, had successfully kept Kyiv’s war planning secret — and not just from the Russians. Neither Washington nor many senior officials in the Zelensky Administration knew Zaluzhny’s blueprint. “We were pessimistic about Ukraine holding out in part because the Ukrainians didn’t share any of their preparations or planning with us,” a senior Pentagon official told me later. “And the preparations and plans that they did share with us were military deception.”

Western governments were stunned by the speed of the Russian advance and mindful of Russian president Vladimir Putin’s warnings not to interfere. Johnson called Zelensky with an offer to arrange an escape, one of several such calls that the Ukrainian president received. One possibility, Johnson explained, was to set up a Ukrainian government-in-exile in London, the way a Polish government-in-exile had been established there after the Nazi and Soviet invasions of 1939. Zelensky asked for weapons instead.

Poland, perhaps because of its history, was the only nation that didn’t despair in these early hours. Rerouting his connecting flight, Kuleba attended Poland’s national security council meeting on February 24, the day of the invasion. The Polish Government, like other Nato members, had been told by the alliance’s intelligence that a swift collapse of the Ukrainian state was near certain. Still, Warsaw refused to give up. Immediately, Poland sent several truckloads of ammunition and heavy weapons. “The Poles believed in us more intuitively than fact-based because all the facts spoke against us at the time,” Kuleba recalled.

The mood was very different in other European capitals. “Nobody was giving the Ukrainians any chances,” Johnson said. “If this is going to happen, the best thing is that maybe it should happen quickly,” a senior aide to German chancellor Olaf Scholz told him at the time.

In his speech on the day of the invasion, Putin threatened unimaginable consequences should the West try to help Kyiv. And, since the first days of the war, the White House’s overriding priority had been not to overstep Russia’s “red lines” and provoke a direct confrontation between Moscow and Nato — especially a nuclear one.

Putin’s admonishments worked, to a significant extent. In the months to come, the United States and its partners held back from supplying Ukraine with Western-made capabilities at a time when they would have had the biggest effect, and prohibited Kyiv from using Western weapons to strike military targets on Russian soil. By the time many of these Western systems did arrive, in the second year of the war, Russia had built up defences, mobilised hundreds of thousands of troops and switched its industries to wartime footing. The best window of opportunity for a clear and quick Ukrainian victory had disappeared.

I went to see Zelensky in his fortresslike office in Kyiv in July 2022. Ukraine had survived, repelling the initial Russian onslaught on Kyiv, but was bleeding men as Russia refocused on seizing the eastern region of Donbas, where the conflict began in 2014.

Encouraged by Kyiv’s military abilities, the Biden Administration and Western allies had just started supplying outgunned Ukrainian forces with Western artillery. It was indispensable: Ukraine’s own Soviet-standard ammunition, used to repel the Russians around Kyiv, had almost run out. Still, Ukrainian requests for Western tanks, fighting vehicles, combat aircraft or Patriot air defences kept getting dismissed out of hand — mostly because of fears about Moscow’s reaction.

Zelensky was grim-faced, chronically tired and visibly aged. I had last met him 2½ years earlier at a banquet in a Kyiv art space. At the time, the former comedian performed a short stand-up routine and deferred thorny questions about his relationship with Donald Trump to the guest of honour at his table, whom he kept calling “the real president”: actress Robin Wright, aka President Claire Underwood in the television show House of Cards.

That quick smile, the sparkle in the eyes and the desire to please the crowd were gone. Again and again, Zelensky returned to the horrendous damage inflicted by Russia on the Ukrainian people. Only a fraction of the many thousands of deaths went reported, he noted. “You have an explosion in a city centre, and 11 people are missing. What does it mean? This means that nothing is left of these people, nothing at all.” He waved his hands. “Children without limbs, children without heads ...”

By failing to seize Kyiv in March, Putin had already suffered a strategic military defeat in Ukraine, Zelensky believed. “He opened his mouth like a python and thought that we’re just another bunny. But we’re not a bunny and it turned out that he can’t swallow us — and is actually at risk of getting torn apart himself.”

Zelensky showed his exasperation with Washington tying itself in knots of self-imposed red lines. He dismissed fears of Russian escalation as unwarranted. After all, as Ukrainian officials kept telling their American interlocutors, Russia had already used all its weapons except the nuclear bomb on Ukraine. What else could it do?

The United States, however, kept taking Russian nuclear warnings seriously. “The Venn diagram between our and Ukrainian interests overlaps about 85 per cent, but that remaining 15 per cent is pretty important,” a senior Pentagon official told me. “The Ukrainians are already fighting for their existence. But the United States has a special obligation to avoid a nuclear war that would end all life on Planet Earth for ever.”

As I talked to Zelensky, Ukrainian, US and British commanders were meeting at a US base in Wiesbaden, Germany, to figure out precisely where the Ukrainians should launch their attempt to regain Russian-occupied land — at that point nearly a quarter of the country.

From Kyiv, Zelensky and Zaluzhny advocated for a push to the Sea of Azov in the Zaporizhzhia region that, if successful, would sever Russia’s “land bridge” to Crimea and deprive Moscow of its biggest prize in the war. After months of losses, the Russians were on the back foot, with as few as 100,000 combat-capable troops left in Ukraine. Unwilling to admit that his “special military operation” was not going according to plan, Putin rejected his generals’ calls to mobilise reservists. Russia was at its weakest. A better chance to strike a decisive blow might not present itself. What Ukraine needed to succeed, Zaluzhny calculated, were about 90 additional howitzers and adequate ammunition, according to his aides.

It wasn’t a huge ask, but the allies weren’t convinced. The Ukrainian military had not yet demonstrated any capacity for offensive operations, especially large ones involving complex co-ordination between multiple brigades, the US advisers said. They urged a more modest operation in Kherson.

Zaluzhny disagreed. “We must attack where we should, not where we can,” he argued, according to his aides. But without the requested package of US weapons and ammunition, the Zaporizhzhia push was impossible. The Ukrainians focused on Kherson and Kharkiv.

In late September 2022, I followed Ukrainian troops into the just-liberated city of Kupyansk, which had served as the capital of the Russian-occupied part of the Kharkiv region. Russian defences had crumbled overnight in Ukraine’s most successful offensive of the war so far. US officials were stunned — and worried.

The big square in front of the municipality was empty, except for two Ukrainian troopers who ran from building to building to evade Russian snipers. Several Russian flags were scattered in the dirt, some half-burnt. A billboard with the words “We are a single people with Russia!” was still hanging on the façade. Abandoned in haste, this had been the inner sanctum of the Russian occupation administration.

The Russians had left behind hundreds of copies of IDs of Kupyansk citizens who had come to collect subsidies or to apply for Russian passports. Some of the rooms were full of brand-new textbooks and teaching aids written in Russian, still wrapped in cellophane and meant for the new schools that were supposed to supplant Ukrainian education. I was surprised by the sheer quantity of paperwork left behind. Minutes of meetings, agendas, guidelines, allocations were spilling from cupboards. The Russian bureaucratic machine had been preparing for permanence.

Joining a Ukrainian patrol, I advanced around the remains of burnt-out Russian tanks and armoured vehicles, past the anti-Russian graffiti that testified to local Ukrainian resistance, and past the bloated remains of a Russian soldier. He was lying in a pool of dirty water under a billboard advertising a Ukrainian fish-processing company that proclaimed, “We value everyone, we work with the best!”

The soldiers belonged to Ukraine’s International Legion, with Ukrainian officers and troops made up from foreign volunteers. I asked one of the legionnaires, a Tennessee medic who went by the call sign “Doc”, why he had decided to come fight for Ukraine. He told me it was the images of destruction wrought by Russia that he had seen on TV. “Everybody saw these pictures. For me, it wasn’t a choice, really. If I didn’t do this, I would have hated myself,” he replied. “I just don’t like fascists and I don’t like people raping and murdering.”

A white truck with Russian military “Z” markings spray-painted on the sides in orange sat in the middle of the road, its windows shot out and two corpses spilling out from the cabin. A tank without a turret smouldered near by, alongside a stack of carbonised cadavers. I took a photo with the squad of legionnaires. Within a few weeks, some of these men were killed.

A pick-up truck sped past. In the back, a Ukrainian soldier winked and raised a thumb. There was a black bundle at his feet, and it took a minute to realise that it was a person. From the back, it seemed like a hunched old woman. It was only once I approached that I grasped it was a wounded Russian soldier.

The Ukrainian escorts asked the blindfolded prisoner where he was from.

“Belgorod,” he said.

“Ah, neighbours! Have you been to Kharkiv before the war?”

“Of course,” the Russian replied. It used to be common for the people of Belgorod to visit the much bigger Kharkiv on weekends. The two cities were only an hour and a half’s drive apart, back when there was no war and when the border was easy.

“Did you see any Nazis there?” the Ukrainian asked, squinting. Putin’s declared goal of the war, after all, was Ukraine’s “denazification”.

“No,” the Russian mumbled.

“Then why the f**k did you come here to fight?”

There was no reply.

That week, as Russian armies fled in disarray, a stern-faced Putin delivered a speech to the nation. Ukraine’s Donetsk, Luhansk, Zaporizhzhia and Kherson regions — none of them fully controlled by Moscow — would henceforth become unalienable parts of Russia.

Once again, he reminded the world of Moscow’s nuclear weapons. “In case of a threat to the territorial integrity to our nation, to defend Russia and our people, we will, without a question, use all the means available to us,” he warned. “This is not a bluff.”

The war, he added, was no longer just against the regime of Ukraine but against “the entire military machine of the collective West”. To win, Russia needed more troops. At least 300,000 more.

Within hours, Russian recruitment offices started rounding up men. Hundreds of thousands of Russians stampeded to flee the country through the few available exit routes. Flights to the few international destinations still linked with Russia sold out in a matter of hours. That week, the cheapest one-way economy ticket from Moscow to Dubai cost nearly $8,000.

Ukraine called Putin’s nuclear bluff. In the following weeks, Kyiv pressed its offensive into areas that Moscow now considered Russian soil, taking the city of Lyman in Donetsk and then Kherson — the only regional capital that Russia had occupied since the full-scale invasion.

But in Washington, fears of a Russian nuclear escalation reached their highest point that week. According to US intelligence estimates, Putin was likely to consider a nuclear strike under three scenarios. One was a major attack on Russia proper, especially with Nato involvement. Another was the possibility of losing physical control over Crimea. And the third, according to a senior Pentagon official, was a Ukrainian battlefield victory “that would completely and totally shatter the Russian military, such that the Russian state would sense an existential threat”.

Crumbling in Kharkiv and Kherson, the Russian military seemed on the verge of such a collapse in the autumn of 2022 — a consideration that, at that critical moment, again tempered US assistance to Kyiv. The Biden Administration reached out to Moscow to de-escalate.

National security adviser Jake Sullivan went on American television networks. “We have communicated directly, privately, to the Russians at very high levels that there will be catastrophic consequences for Russia if they use nuclear weapons in Ukraine,” he said.

By late November 2022, the Ukrainian offensive had run out of steam. There was no massive resupply of artillery ammunition, and Kyiv’s pleas for Western tanks and fighting vehicles kept getting turned down. Meanwhile, Russia’s new commander for the war, General Sergei Surovikin, had hundreds of thousands of fresh troops at his disposal. Ukraine’s advantage in manpower was over. Surovikin ordered these men to spend the winter digging, creating nearly impregnable fortifications along the entire front line.

All the hardware that Ukraine was begging for in 2022 — Leopard and Abrams tanks, Bradleys and Strykers, and Patriot batteries — was eventually provided the next year. “A mountain of steel,” is how US officials termed it.

But, by then, it was a different war. The Ukrainian offensives of 2023 gained little ground against an entrenched, prepared and more numerous enemy. Putin’s nuclear brinkmanship had gained him time — not just to prevent a military collapse, but also for indispensable military aid to Ukraine to get caught up in the United States’ own domestic politics.

• Yaroslav Trofimov is the chief foreign affairs correspondent of the Wall Street Journal. This article is adapted from his new book, Our Enemies Will Vanish: The Russian Invasion and Ukraine's War of Independence